Update service.py to support graceful Service shutdown
Update service.py to support graceful Service shutdown

This change backports the latest version of service.py from oslo-incubator.
The latest version supports graceful shutdown for the Service class, which is
needed to enable graceful shutdown for the Ironic Conductor.

Change-Id: Ide6b549aa64333b7781f3525c6d2ccb8762402bf
Partial-Bug: #1382698
(cherry picked from commit dd542195750a19b16a6b1a7cf89d45c70ef2ae61)
debayanray/ironic_backup