[PoC] Allow specifying a custom access token - Track 2 approach

[jiasli]

A rework of #12:

• Remove the usage of msrest.authentication.BasicTokenAuthentication
• Use Track 2 SDK get_token protocol

Instead of monkey-patching get_login_credentials, we make _get_mgmt_service_client support custom credential natively.

But if we implement EnvironmentCredential (Azure#10241), custom credential support may not be necessary since we can combine the logic into EnvironmentCredential.