Install glance images before starting Nova

The docker driver for Nova needs a registry service to be running.
It is being run inside a container using an image -- that image must
be downloaded. The registry service must be started via
nova_plugins/hypervisor-docker, but this is presently called
before Glance's image download.

The reordering is being done such that Glance may download the
registry image, but prior to starting Nova such that "hypervisor-docker"
may have an image downloaded and available to launch the registry.

This change should cause no negative effects on other hypervisors.

Change-Id: I7bccb42517e4c6187f2a90c64f39cda4577f89a3
blueprint: docker-glance-uploads
Merge "Install glance images before starting Nova"