Enhancement: Support selecting the Azure Could (WABS storage)

[normalerweise]

The Azure Cloud as a whole consists of several clouds e.g. Public (default), US Government, Germany....
Today wal-e supports to store backups in the public cloud only.
To support other clouds, too corresponding configuration should be introduced.

[normalerweise]

The feature is supported by the underlying Azure Python SDK
via the endpoint_prefix parameter passed to the BlobService constructor.

The values corresponding to the different clouds could be found here:
https://docs.microsoft.com/en-gb/azure/storage/common/storage-powershell-independent-clouds#endpoint-suffix

I have an implementation available here: https://github.com/pulseshift/wal-e/tree/feature/select_azure_cloud
In my opinion this is complemented by #378 which should be merged first,
as I needed both fixes to use wal-e with a storage account inside Azure Germany