Ensure that pbr>=0.5.10 is installed

A bug in PBR levels less than 0.5.10 causes the
/usr/lib/python2.X/site-packages/cinder/db/sqlalchemy/migrate_repo/migrate.cfg
file to not be properly installed.  This causes 'cinder-manage db sync'
to fail due to the missing file.

This change sets the required PBR level to >=0.5.10,<0.6 to avoid the problem.
In the case that user already has a 0.5.X level of PBR installed that is less
than 0.5.10 it should be updated rather than encountering issues as our
test system did.

Change-Id: I532158277a4acbb2f8cb75d03c91097e6d7e0312
fixes: bug 1182203
Merge "Ensure that pbr>=0.5.10 is installed"