nova: support setting scheduler_host_manager

Adds support for setting scheduler_host_manager if
it is set via heat metadata. Previously this
value was hard coded based upon the use of
either Ironic or Nova baremetal.

The motivation here is to be able to use the new Ironic driver
location in the Nova tree.

Change-Id: Id4ca7f90cd85129727e65d0d09f274f3a00d4ebe
Merge "nova: support setting scheduler_host_manager"