Allow to load secrets from azure credential file

[mkaszub]

This feature introduce loading of secrets from credential file that may be crated by calling azure cli.

Azure credential file may be crated by calling:

az ad sp create-for-rbac --sdk-auth > credentials.json

chaostollkit extension will pick up credential file when:

• no secrets are defined in experiment file
• path to the credential file is stored in environment variable AZURE_AUTH_LOCATION

More info about azure credential file may be found: https://docs.microsoft.com/en-us/azure/developer/python/azure-sdk-authenticate

This should solve as well #58

[mkaszub]

@bugra-derre could you take a look?

[bugra-derre]

Please add your changes in a short sentence into the CHANGELOG.md