Explicitly pip install pbr before its installed as dependency

When a pypi mirror is used with certain conditions installing pbr as a
dependency fails (see bug
https://bugs.launchpad.net/tripleo/+bug/1293812).
This is a temporary fix until the underlying cause is resolved.

Change-Id: Icfdaa20dc4471fe62a5e29e618d00779e31cbc72
Merge "Explicitly pip install pbr before its installed as dependency"