Remove the mongo auth warning during tests

pymongo print a warning on each mongodb connection because the url used
in tests don't have credentials, since all mongodb tests are running in
a different mongodb databases, this print many annoying line like this one:

UserWarning: database name or authSource in URI is being ignored. If you
wish to authenticate to ceilometer_90d94586954643e8811d5ecc6bacf912, you
must provide a username and password.

This change suppress this line during testing.

Fixes bug #1210674

Change-Id: Iecabca621c2fbaf15e644f2fca66b4e3b5bd04e7
Merge "Remove the mongo auth warning during tests"