heat engine : LoadBalancer resource delete nested stack

Ensure LoadBalancer resource deletes the underlying nested
stack when the owning stack is deleted - needs to inherit
from stack.NestedStack as this implements handle_delete()

fixes bug 1097394

Change-Id: I2002bed361511a495cbdf38cad7b61e148a68ba2
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat engine : LoadBalancer resource delete nested stack"