Update oauthlib requirement to at least 0.6

The oauthlib requirement was not limited. A recent change to
Keystone doesn't work with versions of oauthlib < 0.6.

Change-Id: Ied4d1aea8fba28ec739dbbdc2f5a6ea2194d1674
Closes-Bug: #1270909
Merge "Update oauthlib requirement to at least 0.6"