Add pbr to devstack.

It's being used in all of the core projects, so we should install it
at the start so we can ensure that we don't break everything all at
once.

Change-Id: I326d724264803e88315ee9e40f4634836baf6e0b
Merge "Add pbr to devstack."
Merge branch 'master' of github.com:openstack-dev/devstack