setting up neutron plugin file in /etc/default/neutron-server to contrail plugin, this is needed for icehouse as stock neuton-server is used. Havana has locally build neutron-server where it is populated at build time

Change-Id: I650a72cd4ce0118f18f82e894eaca270c2abe27e
Merge "setting up neutron plugin file in /etc/default/neutron-server to contrail plugin, this is needed for icehouse as stock neuton-server is used. Havana has locally build neutron-server where it is populated at build time"