Add oauthlib library to replace oauth2

The current oauth library (oauth2) is unmainted and has security
issues, as mentioned in:
https://bugs.launchpad.net/keystone/+bug/1240382

The better choice would be to use oauthlib.

This patch is to add oauthlib. When we fully remove oauth2 from
keystone, we can remove oauth2 from requirements.

Change-Id: I1c14c71fb37d8e164585d785c7755eccb4c9c51e
Merge "Add oauthlib library to replace oauth2"