graceful-shutdown: add graceful shutdown into compute

The following commit brings in oslo-incubator service.py which
includes calls to support graceful shutdown.  The common service
class has changed to handle signals and manage Services.  When
a signal is received the Services are stopped which in turn
waits on all child threads (Proxy callbacks) to complete.

The oslo-incubator commit hash that was used to update
nova was b5fba9e40d609d63b4dab9ca351f7e22e9d8b35b.

This allows long running commands to complete prior to killing
the process.  This change only works for SIGTERM and SIGINT
signals.

Unit test was added to verify parent stop method is called in
a shutdown condition.

Change-Id: I21a45862f491530523037632fd70c60cc0153c0a
Implements: blueprint graceful-shutdown
Merge "graceful-shutdown: add graceful shutdown into compute"