AdminUserTag added to JujuConnSuite

The main change here is to add a method to the JujuConnSuite to get the AdminUserTag.

This is just a names.UserTag and currently only returns "admin".

I have a pending branch that makes it so the initial user is not always called admin. This broke a heap of tests.  This is how we can fix the tests first.
The test here is for inappropriate tags. The use of admin is irrelevant to the test.