[r=dimitern] api(server)/uniter: Return tags instead of names
UniterAPI.SubordinateNames() is renamed to
HasSubordinates() and returns (bool, error).
Added a TODO in uniter/modes.go to use it
instead of calling len(u.unit.SubordinateNames) > 0,
and since the other place SubordinateNames() was
called is now handled by DestroyAllSubordinates()
there's no need for it anymore.
Also GetPrincipal() now returns a unit tag, not
a name.
https://codereview.appspot.com/13348050/
R=fwereade