Skip lbaas table creation if tables already exist

This patch makes possible upgrade from Grizzly to Icehouse
in case LoadBalancer service plugin was used in Grizzly. LoadBalancer
tables were introduced in Grizzly therefore they necessarily don't
need to be created for Havana and beyond.

This patch goes to Icehouse stable branch only because of all migration
scripts till Havana were squashed together in Juno release. That means
modified file in this patch no longer exists in current master branch.

Change-Id: I09ed3e4b142ccde44cf2e92184d0d92d898a0284
Closes-bug: 1273355
Merge "Skip lbaas table creation if tables already exist" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse