Add the user- prefix to the users username for all external requests
Moved the user name prefix into an API_USER_TAG constant in the file
Merge pull request #684 from hatched/prefix-user-name

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.