* Read ACS Credentials for Azure Authentication
Supprt a new environment variable: ACS_CREDENTIAL_LOCATION
Expect the value to be the ACS credential filepath, which is the
/etc/kubernetes/azure.json file generated on the ACS nodes.
If the ACS_CREDENTIAL_LOCATION is specified and loaded,
create the Azure Authentication class from its values.
If the AZURE_AUTHENTICATION_LOCATION is specified and loaded,
its values will overwrite the value above.
Refactor the ACI provider and ACI client to be able to override the SPN by environment variable
