Don't start tempest before nova-compute is ready

If nova-compute and nova-conductor are on different nodes
and n-compute is up before n-conductor, n-compute will retry
to register health but there will be increasing wait time
between retries, so make sure n-conduct sees available compute
nodes before starting tempest.
also added a parent http proxy to speed up installation and
a dependency for tempest.

Change-Id: Ica46c66bc8b7589915490555d86674eb49c0db1c