Wait till message handling is done on service stop

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

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

Closes-bug:#1295625
Change-Id: I8556b598f6263a2d4d77e8315950ef1b298aa0a1
Merge "Wait till message handling is done on service stop"