Set nullable=False on tenant_id in apic_contracts table

The cisco_ml2_apic_contracts table has a tenant_id field that is not nullable
since it is the primary key. The HasTenant mixin does not allow setting
nullable=False, so use sa.Column() explicitly for this.

Change-Id: If8e96e76fad0bfcf2bfd71b0223624f1da4517c9
Closes-bug: #1342756
Fix wrong order of tables in downgrade

Heal migration fix bug https://bugs.launchpad.net/neutron/+bug/1336177.
Now table ml2_brocadenetworks has foreign key and downgrade
of 492a106273f8_brocade_ml2_mech_dri fails. To fix this change order
of tables in downgrade is needed.

Closes-bug: #1349810

Change-Id: Ida2ba66d35a952381f213080d417e475e18c196d
Merge "Set nullable=False on tenant_id in apic_contracts table"
Merge "Fix wrong order of tables in downgrade"