Log which filter failed when on log level INFO

If log level is set to INFO,only see a log message about nova-schedule
attempting to build n instances then a log message about setting the
state to error when the scheduler fails to schedule an instance due to
a filter returning 0 hosts.It may be useful to see which filterreturns 0
host for operator.

Close-bug: #1226334

Change-Id: I7564a8ab093895d1f9db357009aac294783a808b

nova/filters.py

@@ -80,9 +80,10 @@ class BaseFilterHandler(loadables.BaseLoader):
                           {'cls_name': cls_name})
                     return
                 list_objs = list(objs)
+                if not list_objs:
+                    LOG.info(_("Filter %s returned 0 hosts"), cls_name)
+                    break
                 LOG.debug(_("Filter %(cls_name)s returned "
                             "%(obj_len)d host(s)"),
                           {'cls_name': cls_name, 'obj_len': len(list_objs)})
-                if len(list_objs) == 0:
-                    break
         return list_objs