Add a list_stacks RPC call.

Add a list_stacks RPC call.

This differs from the old show_stack(None) by not resolving stack data.

heat list does not display any of the resolved stack data. Resolving outputs
can be especially slow since FnGetAtt may block while it calls out
to another service (such as with the quantum resources).

Without this change, running heat list on a single Quantum.template takes 1.8s
vs 0.7s.

In the future, list_stacks can be made to support paging of results.

Change-Id: Iae30338b66912d660f1cf48119893b1ab593b67a