fix(security): Secure commands on namespace deletion

[germainlefebvre4]

Current status: Kubernetes namespaces are deleting using os library.

$ pipenv run bandit -r handler.py

>> Issue: [B605:start_process_with_a_shell] Starting a process with a shell, possible injection detected, security issue.
   Severity: High   Confidence: High
   Location: handler.py:56
   More Info: https://bandit.readthedocs.io/en/latest/plugins/b605_start_process_with_a_shell.html
55              if not DRYRUN:
56                  os.system("kubectl delete namespace {}".format(ns_name))
57

Solution:

• Use kubernetes client library