Enable mutable config in kingbird

New releases of oslo.config support a 'mutable' parameter to Opts.
oslo.service provides an option here Icec3e664f3fe72614e373b2938e8dee53cf8bc5e
allows services to tell oslo.service they want mutate_config_files to be
called by passing a parameter.

This commit is to use the same. This allows kingbird to benefit from
I1e7a69de169cc85f4c09954b2f46ce2da7106d90, where the 'debug' option
(owned by oslo.log) is made mutable. we should be able to turn debug
logging on and off by changing the config.

tc goal:
https://governance.openstack.org/tc/goals/rocky/enable-mutable-configuration.html

Change-Id: If25111f52da3590bf688cc96b9346a9b978a485c

kingbird/api/app.py

@@ -83,7 +83,8 @@ def serve(api_service, conf, workers=1):
     if _launcher:
         raise RuntimeError(_('serve() can only be called once'))
 
-    _launcher = service.launch(conf, api_service, workers=workers)
+    _launcher = service.launch(conf, api_service, workers=workers,
+                               restart_method='mutate')
 
 
 def wait():

kingbird/cmd/engine.py

@@ -45,7 +45,8 @@ def main():
     srv = engine.EngineService(cfg.CONF.host,
                                consts.TOPIC_KB_ENGINE)
     launcher = service.launch(cfg.CONF,
-                              srv, workers=cfg.CONF.workers)
+                              srv, workers=cfg.CONF.workers,
+                              restart_method='mutate')
     # the following periodic tasks are intended serve as HA checking
     # srv.create_periodic_tasks()
     launcher.wait()