Fixes the following vet reports:
builder/openstack/artifact.go:44: arg a.ImageId for printf verb %d of wrong type: string
builder/openstack/step_wait_for_rackconnect.go:24: arg server for printf verb %s of wrong type: *github.com/mitchellh/gophercloud-fork-40444fb.Server
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.