Making the primary interface configurable

It would help quite a bit to make the primary interface that is used
for service endpoints and the like configurable. Specifically, I would
like to in order to build out some Vagrant configurations.
Cleanup 2 missing areas for primary_interface

Vagrant defaults should not use ansible_eth0 directly nor should
cinder.conf.
Merge pull request #225 from craigtracey/interface

Making the primary interface configurable