Skip auth tests if auth not enabled PYTHON-556

This change does a few things:

1. In anticipation of the server defaulting to a "disabled" mode for
auth, only run authentication related tests if auth is enabled.
2. Attempt to clean up users in a finally block, even if tests failed.
3. Look for auth settings in "parsed" from getCmdLineOpts in case the
cluster was started with config files.
4. Remove useless remove_all_users calls at the beginning of tests - if
users were added in a previous test and we forgot to clean them up
these are going to fail no matter what when auth is enabled.