marvin: enable the nose-marvin plugin using the --with-marvin directive

enable the marvin plugin when the --with-marvin directive is given to nose. The
enableOpt value is set from the command line directive and self.enabled
(True|False) determines whether marvin's tests will run.  By default
non-default plugins like marvin will be disabled"

This fixes regression from 03830c5

Signed-off-by: Prasanna Santhanam <tsp@apache.org>