Default security group table

Default security group table

This change prevents the race condition by enforcing a single default
security group via new table default_security_group. It has tenant_id
as primary key and security_group_id, which is id of default
security group. Migration that inroduces this table has sanity check that
verifies that there is no duplicate default security group in any
tenant.

This idea has come up from discussion in comments to
https://review.openstack.org/135006

DocImpact

Closes-bug: #1194579

Change-Id: Ifa8fbddd22bce4c50836cf443ebe10dff37443ef