Fix compute_driver and scheduler_host_manager in install-guide

This patch updates the install-guide to use the Ironic driver in the
Nova tree instead of the one in the Ironic tree, as I am sure the
the one in Ironic's tree is about to be deleted.

Change-Id: Idd526f8b36eee3501e3a955414c419150285993b
Merge "Fix compute_driver and scheduler_host_manager in install-guide"