Make auth in MgoSuite optional, disabled by default

The Juju tests mostly don't care about authentication.
By disabling authentication by default, segregating
the tests that do care (i.e. explicit checks for
authentication/authorisation) and enabling authentication
for them, then we can easily run with either Mongo 2.4 or
2.6 despite the changes made to security between the
versions.
Merge pull request #28 from axw/mgo-enableauth-option

Make auth in MgoSuite optional, disabled by default