This adds two config options that we need in order to successfully build
our Rackspace images.

First, a boolean `rackconnect_wait` option which waits for the
RackConnect metadata to appear.

Second, an `ssh_interface` option, for rackconnect users who have more
prohibitive firewalls on the 'public' interface and want to ensure all
traffic to the server goes over the 'private' one.

Finishes #952.