Don't use ML2 parameters for other plugins

CONFIG_NEUTRON_ML2_MECHANISM_DRIVERS parameter must not be used
when monolithic Neutron plugins are being installed. This patch
is fixing installation failures for OVS and LB plugins.

Fixed Gerrit default branch for icehouse branch

Change-Id: Id6e7cdf2ab46a024fb78f74c77cff8f847ada038
Fixes: rhbz#1119473
(cherry picked from commit ec5cf0ef5cc575b72db4477f3113957f8f6b4906)