Remove glance_notifications from notification_topic

The default notification_topic in Glance has been changed from
'glance_notifications' to 'notifications' in grizzly-2.

Ceilometer was configured to also listen on the
'glance_notifications' notification_topic in addition to
'notifications' to patch this bug. It shouldn't anymore.

Change-Id: I3d797c7e849319a1e20ef3ecbf158e8b8579d450
Closes-bug: #1273876
Merge "Remove glance_notifications from notification_topic"
Allow log_dir to be set to false in order to disable file logging

This backward compatible commit allows a user to disable logging to
a directory for example if using syslog logging.

Change-Id: I8b496c311b9f26cab64c38d1cfd545784dfc7cab
Related-bug: #1269482