add azure KMS as another option in refs

[anirban1c]

Fixes issue # Add azure kms as another backend as described here https://github.com/Azure/azure-sdk-for-python/tree/master/sdk/keyvault/azure-keyvault-keys#cryptographic-operations

this requires a key vault with a key and a service principal with - GET, ENCRYPT and DECRYPT added to it

Proposed Changes

add another module azkms update cli with --vault option add in azure pip dependencies

TODO: add a test module for azkms

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[anirban1c]

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[uberspot]

----------------------------------------------------------------------
ImportError: Failed to import test module: tests.test_binary
Traceback (most recent call last):
  File "/opt/python/3.7.1/lib/python3.7/unittest/loader.py", line 434, in _find_test_path
    module = self._get_module_from_name(name)
  File "/opt/python/3.7.1/lib/python3.7/unittest/loader.py", line 375, in _get_module_from_name
    __import__(name)
  File "/home/travis/build/deepmind/kapitan/tests/test_binary.py", line 29, in <module>
    from kapitan.cli import main
  File "/home/travis/build/deepmind/kapitan/kapitan/cli.py", line 37, in <module>
    from kapitan.refs.secrets.azkms import AzureKMSSecret
  File "/home/travis/build/deepmind/kapitan/kapitan/refs/secrets/azkms.py", line 29, in <module>
    from azure.keyvault.keys import KeyClient
ModuleNotFoundError: No module named 'azure.keyvault.keys'

Travis is failing because No module named 'azure.keyvault.keys' Is there a pip dependency missing from requirements.txt? I might be wrong but https://docs.microsoft.com/en-us/python/api/overview/azure/key-vault?view=azure-python mentions using from azure.keyvault import KeyVaultClient instead?

[pievalentin]

Would be interested in seeing this merged. Is forking and doing a PR good practice?

[uberspot]

If you maintain the other authors' commits I don't see why not. :)