Lambda hook breaks if no bucket is defined in the options or in the config

[danielkza]

It's possible to have no S3 bucket available if none is specified in the hook options, and the config deliberately sets stacker_bucket to the empty string.

The Lambda hook needs to check for that condition and fail with an explanatory error message instead of blowing up:

[2018-10-22T20:49:14] lambda: using default bucket from stacker: None
[2018-10-22T20:49:14] Method stacker.hooks.aws_lambda.upload_lambda_functions threw an exception:
Traceback (most recent call last):
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/util.py", line 403, in handle_hooks
    result = method(context=context, provider=provider, **kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/hooks/aws_lambda.py", line 501, in upload_lambda_functions
    ensure_s3_bucket(s3_client, bucket_name, bucket_region)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/util.py", line 514, in ensure_s3_bucket
    s3_client.head_bucket(Bucket=bucket_name)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 314, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 586, in _make_api_call
    api_params, operation_model, context=request_context)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 619, in _convert_to_request_dict
    api_params, operation_model, context)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 648, in _emit_api_params
    params=api_params, model=operation_model, context=context)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/hooks.py", line 227, in emit
    return self._emit(event_name, kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/hooks.py", line 210, in _emit
    response = handler(**kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/handlers.py", line 217, in validate_bucket_name
    if VALID_BUCKET.search(bucket) is None:
TypeError: expected string or buffer
Traceback (most recent call last):
  File "/usr/lib64/python2.7/pdb.py", line 1314, in main
    pdb._runscript(mainpyfile)
  File "/usr/lib64/python2.7/pdb.py", line 1233, in _runscript
    self.run(statement)
  File "/usr/lib64/python2.7/bdb.py", line 400, in run
    exec cmd in globals, locals
  File "<string>", line 1, in <module>
  File "/home/danielkza/cobli/deploy/cloudformation/venv/bin/stacker", line 3, in <module>
    from stacker.logger import setup_logging
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/commands/stacker/build.py", line 58, in run
    dump=options.dump)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/actions/base.py", line 188, in execute
    self.pre_run(*args, **kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/actions/build.py", line 341, in pre_run
    outline
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/actions/build.py", line 148, in handle_hooks
    context=context
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/util.py", line 403, in handle_hooks
    result = method(context=context, provider=provider, **kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/hooks/aws_lambda.py", line 501, in upload_lambda_functions
    ensure_s3_bucket(s3_client, bucket_name, bucket_region)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/stacker/util.py", line 514, in ensure_s3_bucket
    s3_client.head_bucket(Bucket=bucket_name)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 314, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 586, in _make_api_call
    api_params, operation_model, context=request_context)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 619, in _convert_to_request_dict
    api_params, operation_model, context)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/client.py", line 648, in _emit_api_params
    params=api_params, model=operation_model, context=context)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/hooks.py", line 227, in emit
    return self._emit(event_name, kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/hooks.py", line 210, in _emit
    response = handler(**kwargs)
  File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python2.7/site-packages/botocore/handlers.py", line 217, in validate_bucket_name
    if VALID_BUCKET.search(bucket) is None:
TypeError: expected string or buffer