webadmin: Refactor RoleTreeView

RoleTreeView had too long and complicated method
with too many indentation levels. The patch reduces
the complexity and allows new entities to be added
more easily to the Action Groups tree.

Change-Id: I82431f3e96be13d3815e9e89130f4cd674394bc8
Related-To: https://bugzilla.redhat.com/1002483
Signed-off-by: Moti Asayag <masayag@redhat.com>