Review [1] introduced a regression that reverted the updated value for
scheduler_host_manager made in review [2]. This old value worked fine
prior to newton but causes nova to fail to restart on newton.
This commit also removes the stale nova_scheduler_host_manager variable
which should no longer get used.
[1] https://review.openstack.org/#/c/293315/
[2] https://review.openstack.org/#/c/298450/
Change-Id: Iedf3892d6317cb3566e40460bdbb05d15b4641ae
(cherry picked from commit a902528a24)