Added new variable SDN_PLUGIN to handle gate behavior when using ovs or
linuxbridge.
By default SDN_PLUGIN is set to ovs.
To enable testing linuxbridge, new gate have to be created, with
SDN_PLUGIN set to linuxbridge.
Change-Id: I014307ae497374b5b6fa00946b01fa5d93c3056f
Implements: blueprint support-linux-bridge-on-neutron