Fix pbr to version 2.0.0
Fix pbr to version 2.0.0

* This is to fix a breaking change with the current builds
  that fails when using versions greater than 2.0.0 of pbr.
* We will need to be addressed better in future once openstack
  libraries can work with newer pbr.

Change-Id: I3e8e547af7005ca06fc5bbc6064718b664507d96

catalyst/stacktask