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
Add attributes_schema to CinderVolume

The branch adds descriptions for the attributes of CinderVolume, and
fixes some properties description.

Closes-Bug: #1224349
Change-Id: Ib9bf7aaeb1b4de7141e82e7e09d60f8c2de07de0
Merge "Add config option to limit max stacks per tenant"
Merge "Add attributes_schema to CinderVolume"