Various changes to support Mongo 2.6

Disable auth in Mongo for all except
a handful of tests, which we segregate
and enable authentication for explicitly.
Merge pull request #506 from axw/mongo2.6-compat

Various changes to support Mongo 2.6

Disable auth in Mongo for all except a handful of tests, which we segregate
and enable authentication for explicitly.

Ignore all commits other than 326c231: this builds on top of PR #498.

Requires https://github.com/juju/testing/pull/28