Add pinned dependency for pbr

[rgbkrk]

Pin dependency for pbr to workaround current issue with pbr.

[coveralls]

Coverage remained the same when pulling c4afc33 on rgbkrk:pbr_pin into 5555011 on rackspace:working.

[EdLeafe]

I'm not sure if we want to do this for what might be a temporary problem. Let me dig into it more.

[rgbkrk]

I completely agree that we should fix the upstream dependency in pbr, as this will be temporary.

Any new installations until then though will likely have this same problem, including affecting any automated installations (even if pinned, due to pinning python-novaclient>=2.13.0).