Explicitly install pbr via pip

When a pypi mirror is used with certain conditions installing pbr as a
dependency fails.  This is a temporary fix until the underlying cause
is resolved.

This is also required for a wheels based pip mirror to work.

Change-Id: Ia08eb16b96d9897fa2f86239ea264b145ef0b9b5
Closes-Bug: #1293812
Merge "Explicitly install pbr via pip"