Fixed nova notifier unit test

This fixed the bug #1221033 caused by the latest nova chagnes.

Temproarily enlarge the max allowed connection to mongoDB as a
workaround for bug #1218488. That bug should be addressed by
https://review.openstack.org/44465

Change-Id: I49d82bcc2c1ac6481b0f5d321db2f70549a95d55