Add pbr to dependency list

We use pbr to install stevedore, and because of the way setuptools deals
with install-time requirements we also need to include it as a runtime
dependency to ensure it is installed by pip and not easy_install.

Change-Id: I0a39bb50218a1a7cdb2d42fb474a9b8f6d6ac32a
Closes-Bug: #1384919