[API] Support per-tenant max loadbalancers

Note: Schema altering change

Adds a new table that allows maximum number of load balancers to
be set per tenant ID. If a value is not set, the global value is
used. The per-tenant value always overrides the global value.

Change-Id: Ib7911742510756e34a4ef0ecf9380abb2a759e8e