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
Merge "Set nullable=False on tenant_id in apic_contracts table"