Enable mutable config in blazar

New releases of oslo.config support a 'mutable' parameter in Opts.
oslo.service provides an option [1] which allows services to tell it
they want mutate_config_files to be called by passing a parameter.

This commit is to use the same approach. This allows Blazar to benefit
from [2], 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 and sending a SIGHUP signal to blazar-manager.

However, please note that blazar-manager currently doesn't work
correctly after receiving a SIGHUP. As a result the mutable config is
not yet usable. Operators should continue restarting blazar-manager
after changing blazar.conf.

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

[1] https://review.openstack.org/263312/
[2] https://review.openstack.org/254821/

Change-Id: Ieea9043b6f3a28dc92717680585614a68227120e

blazar/cmd/manager.py

@@ -37,7 +37,8 @@ def main():
     notifier.init()
     service.launch(
         cfg.CONF,
-        manager_service.ManagerService()
+        manager_service.ManagerService(),
+        restart_method='mutate'
     ).wait()