Add pbr to installation requirements
Add pbr to installation requirements

Add pbr to the list of installation requirements so that it is installed
via pip before this library is installed, instead of with easy_install.
This avoids issues like Bug #1384919, and ensures that projects that use
this library as a dependency are properly installed.

Change-Id: I6c155370dbd01fe4748d5137bdf288e8d3e1a67e

openstack/oslo.concurrency,varunarya10/oslo.concurrency,JioCloud/oslo.concurrency