Wait until service thread is done on service stop

Wait until service thread is done on service stop

In order to be able to perform graceful shutdown of services, we need
to wait for all the current service threads to finish before exiting
from the service process.

Oslo.service provides this facility through it's wait() method, so we
need to call it in Cinder Service stop() method which gets called when
graceful shutdown is requested by sending the process SIGINT or SIGTERM.

Partial-Bug: 1464822
Change-Id: I972b80251cebe353f9d89ff4e05db8d029ef5e73