Don't read security groups again

This fix is the cherry pick of
https://github.com/Juniper/contrail-neutron-plugin/commit/b1ab30b62b4b8465ae9ddb16bad743e5f2a9fb4a

Change-Id: Iced6d3d9174f9319d02aec482229c2ff146de6aa
Merge "Don't read security groups again"