Environment owner and new environment.

Add owner and server uuid to the environment document.

The server uuid is added to allow us to be able to refer to environments that are not local.  This is a prelude to support for cross environment relations.  From the outside of a juju environment we have started using the pair of uuids to describe a juju environment (env uuid, server uuid).

There is a drive by fix to the usermanager which I thought was on the other branch, but it got merged already.
env.EnvironTag() returns an EnvironTag, and env.Tag() returns a names.Tag