Shut the heat-engine after all threads finished

Currently heat-engine do shutdown immediately when SIGTERM received.
At that time, a stacks in middle of processing would terminate forcefully.
It's not graceful.

This patch aims to implement graceful shutdown with following methods.

  * Close rpc connection at first for preventing new requests arrived
    after SIGTERM received.
  * Stop stack processing with graceful option.
    The graceful stop functionality is provided by oslo-incubator.
  * Then terminating process.

Change-Id: I8689b830774f7916febb59aca00979d92c0448b5
Closes-bug: #1304244
Merge "Shut the heat-engine after all threads finished"