juju/testing: don't remove admin user in JujuConnSuite

We no longer enable mongo auth in our tests, so there's
no need for the "soft resetting" anymore.
Merge pull request #688 from axw/jujuconnsuite-dont-remove-admin

juju/testing: don't remove admin user in JujuConnSuite

We no longer enable mongo auth in our tests, so there's
no need for the "soft resetting" anymore.