repo.pp: Support havana

Both RDO and UCA have added support for havana; include it in
openstack::repo.

Also, add proper support for Havana in repo/rdo.pp, and import package
signing key, sourced from:
https://raw.github.com/redhat-openstack/rdo-release/160c4183784382d311cde6b7d0dbe1bafb6731a1/RPM-GPG-KEY-RDO-Havana

The default for $release should switch to 'havana' once the Havana
release has dropped.

Change-Id: I0cbcc341235e0d73a0c49eec1254288d825fa787