refactor(contrib/rackspace): create private network on Rackspace

Previously, we were creating servers using the default Rackspace
ServiceNet as the private network. This means that they are feasibly
accessible by other servers on that network. This commit ensures
we are creating a new, private network for deis and not connecting
to the ServiceNet any longer.

closes #891
Merge pull request #892 from deis/rs_private_network

refactor(contrib/rackspace): create private network on Rackspace