Service group drivers forced_down flag utilization

Actual usage of the forced_down flag in case of usage of is_up() method. If
forced_down flag is set (i.e. have value True) than is_up unconditionally
return False - means service is down.

Partially-Implements: blueprint mark-host-down

Change-Id: Ia8ae49c5a38b217f19632a1ac40343ec9e608796

nova/servicegroup/api.py

@@ -88,6 +88,9 @@ class API(object):
         """Check if the given member is up."""
         # NOTE(johngarbutt) no logging in this method,
         # so this doesn't slow down the scheduler
+        if member.get('forced_down'):
+            return False
+
         return self._driver.is_up(member)
 
     def get_all(self, group_id):