Fix notification for NotImplemented record_events

With mongodb as storage backend, notification-to-sample will be broken by
NotImplementedError of record_events() method if we have set
"store_events" option as True in ceilometer.conf.

This patch catch the NotImplementedError and enable the
notification-to-sample process.

Change-Id: I7ace5a8581518fa56e58abca2e377b513ff38675
Closes-Bug: #1320420
Merge "Fix notification for NotImplemented record_events"