Replacing python-oauth2 by oauthlib

This patch replaces the old, unmaintained python-oauth2 library
by the better suited oauthlib in keystone oAuth modules.

The library switch comes with two notable changes in terms of use:

* the client must set the callback uri to 'oob' (out-of-band)
  explicitly when requesting a Request Token
* the requested_project_id header is not included in the signature
  anymore, in compliance with the oAuth1 spec.

Closes-Bug: 1240382
Change-Id: Ie553830cc80075aa818e719604e6bc4c754d2ae3
Merge "Replacing python-oauth2 by oauthlib"