switch to using a subtree for openstack-common

This is a proposal to track the upstream openstack-common cookbook in a SUSE-Cloud repository where we can make any needed adjustments (such as backporting to ruby 1.8). We then use that repository as a subtree here.

This pull-request looks awful. You can see it better at:
https://github.com/mapleoin/barclamp-trove/commits/common-subtree It's actually just those three commits from today, but it also adds in the subtree which I guess is what makes github blow up.