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
Unpin keyring, just skip keyring 2.0

As per bug 1197988, keyring 2.1 has the bug fixed, so unpin keyring.

Change-Id: I9e60e134bed338781cc5a14ee8caae4bc93a538c
Merge "Add oauthlib library to replace oauth2"
Merge "Unpin keyring, just skip keyring 2.0"