Add multi-process support for API services

Implements blueprint multi-process-api-service

This is based on Huang Zhiteng's patch.

This patch adds support for running services as multiple processes. This
is primarily intended to be used with the API service as a way to provide
more concurrency than eventlet can sometimes provide.

A SIGTERM or SIGINT signal will cause the parent process to gracefully
terminate the child processes, allowing them to finish processing the
requests currently being processed. The parent will wait for the
children to finish before exiting.

Change-Id: Ie6d6802626eb42d5e64c4167be363fbf6cea2a1b

bin/nova-api-os-compute

@@ -20,7 +20,7 @@
 """Starter script for Nova OS API."""
 
 import eventlet
-eventlet.monkey_patch()
+eventlet.monkey_patch(os=False)
 
 import os
 import sys
@@ -42,5 +42,5 @@ if __name__ == '__main__':
     logging.setup()
     utils.monkey_patch()
     server = service.WSGIService('osapi_compute')
-    service.serve(server)
+    service.serve(server, workers=server.workers)
     service.wait()