Add graduated oslo code to requirements

This is the first step of the oslo usage cleanup.
The patches to clean-up the local copy of oslo code
will be submitted separately.

Change-Id: Ic59275e78478c328517960f010095b86f0030402
Partially-Implements: blueprint improve-oslo-usage
Merge "Add graduated oslo code to requirements"
Merge branch 'master' of https://github.com/openstack/horizon