Also install python3-pbr
Also install python3-pbr

Some specs that provide a python3 package require it.
This should fix the build of some packages on F21 e.g.
python-stevedor, oslo-config and others.

Change-Id: Id26e0708c7d882c2759940f291767e9faa730164

openstack-packages/delorean,openstack-packages/delorean,openstack-packages/DLRN,openstack-packages/DLRN