Move db migration of ml2 security groups to havana

ml2 plugin is a havana feature. Currently securitygroups table are
created in chain of migration after havana release. It causes db
migration failure when migrating from havana to current head because
securitygroups table is attempted to be created although it was already
created by create_all().

Closes-bug: #1271231
Change-Id: If450bd95de7a5fdfdb2219cfbf7eb0c1323044bb
Merge "Move db migration of ml2 security groups to havana"