[openstack_neutron] fix legacy component detection test

Currently the Neutron plugin will assume it's running on a Neutron
(rather than a legacy Quantum) installation if:

  - '/etc/neutron' exists AND the user specifies quantum=True

Otherwise it's assumed to be a Quantum install. This is clearly
broken; the sense of the 'openstack_neutron.quantum' part of the
test should be inverted.

Fixes #420.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>