Change scheduler configurations to use entrypoints

Full class path style configuration of options scheduler_host_manager
and scheduler_driver are deprecated because of dependent changes. This
commit changes the related configurations to use entrypoints in setting
up nova scheduler in devstack.

Related to blueprint scheduler-driver-use-stevedore
Depends-On: I8c169e12d9bfacdbdb1dadf68b8a1fa98c5ea5bc
Depends-On: I3fd42ead44487a21eb5cfaf5a91209277ce30ad0

Change-Id: Iad96c270073b63719237cf9a9aa1c2dc4daa213a
diff --git a/lib/nova b/lib/nova
index 79bef9b..807fdf6 100644
--- a/lib/nova
+++ b/lib/nova
@@ -96,7 +96,7 @@
 
 # Nova supports pluggable schedulers.  The default ``FilterScheduler``
 # should work in most cases.
-SCHEDULER=${SCHEDULER:-nova.scheduler.filter_scheduler.FilterScheduler}
+SCHEDULER=${SCHEDULER:-filter_scheduler}
 
 # The following FILTERS contains SameHostFilter and DifferentHostFilter with
 # the default filters.