Otherwise, we've seen intermittent "Unable to establish connection"
failures, with the main devstack log reporting things like
2017-07-19 13:54:29.973 -> start proxy service
2017-07-19 13:54:30.082 -> start OSC to store temp url key
2017-07-19 13:54:31.908 -> OSC reports failure
Meanwhile, the s-proxy screen session tells us things like
Jul 19 13:54:31.919988 -> start child worker
Jul 19 13:54:32.206598 -> still loading the WSGI app
... and ports aren't actually bound until *after* the app is loaded.
Add a wait_for_service call to wait for the proxy to come up.
Change-Id: I1a722de31b144797230991700e110353a2d937dd
ab70350403