Define some Cloud Instance constants and the usage pattern of using them

[rayluo]

The new usage pattern is documented in our doc staging site (please search the keyword "authority (str)") in this page.

You can also try the new syntax by pulling its feature branch:

pip install git+https://github.com/AzureAD/microsoft-authentication-library-for-python.git@cloud-instances

This is not a breaking change. The old school, low-level url string input is still supported.

This PR, once agreed upon and merged in, will resolve #221.

[henrik-me]

By default, we will use https://login.microsoftonline.com/common

for CCA it should be tenanted and common should not be allowed?

[rayluo]

It is true that AAD recommend to avoid "common" for confidential client, yet at the same time they can't really block such a usage due to backward compatibility concern.

We are essentially in the same boat here. Historically the input parameter authority is optional and defaults to that ".../common" value. We have, however, long been updated our samples to guide confidential app developers to use their tenant. See here and there. In practice, few people would read our API Reference doc (sad but true :-) ) but they will typically start from our samples. So, I think we are good.

UPDATE: Got some clarification that the "common" is not recommended for acquire_token_for_client(). We will emit a warning for that, in a separate PR #435.