Pass NeutronMechanismDrivers parameter to prepare
Pass NeutronMechanismDrivers parameter to prepare

This change is part of the proposal to use the value of
NeutronMechanismDrivers to determine what neutron images to prepare
(specifically whether the Opendaylight or OVN images should be
prepared).

Change-Id: I1fd78fd1eeb071788fc23ba343915ce7d6d14f02
Partial-Bug: #1794103

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,dprince/tripleo-heat-templates