Explicitly install pbr via pip

When a pypi mirror is used with certain conditions installing pbr as a
dependency fails.  This is a temporary fix until the underlying cause
is resolved.

This is also required for a wheels based pip mirror to work.

Change-Id: Ia08eb16b96d9897fa2f86239ea264b145ef0b9b5
Closes-Bug: #1293812
Ensure the nova element is installable

On a system that has access only to a pypi-mirror without
cached wheels, there a failure to build the overcloud-compute
image.

This is due to the requisite *-dev packages not necessarily
being present on the system.

(With binary wheels, this problem won't surface.)

Correct the nova element to specify its binary dependencies
explicitly.

Change-Id: Iae97ccb63a9fd33d2aa132c0b6c245554cbadf63
Merge "Explicitly install pbr via pip"