mgr: python modules can now perform authorization tests

mgr: python modules can now perform authorization tests

In cases where the python service or individual python modules are
enabled via caps, the module might want to perform finer grained
tests to ensure specific commands are allowed. An example of this is
the 'rbd_support' module limiting access by pools and namespaces.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>