Add the user- prefix to the users username for all external requests

Any time the GUI needs to talk to juju-core it needs the 'user-' prefix to the username which we don't require the user to input when logging in. 
Unfortunately there is some rather odd logic around when these credentials get set. When the env is instantiated we can pass in the user and password so at that point there are no credentials set. It goes to first fetch any credentials, then sets the supplied ones and then sets the credentials using setCredentials. Then later down the tree it calls getCredentials again.