Add config option to limit max stacks per tenant

Tenants will not be able to create more than max_stacks_per_tenant top
level stacks. This should prevent any one tenant from being able to
exhaust a Heat engine's resources.

Fixes bug #1221849

Change-Id: I29be4ecf5d5b9eb44d7ecc01907d08f30ac27990
Merge "Add config option to limit max stacks per tenant"