Make oslo.messaging, magnum, and zaqar reproducible.

Whilst working on the Reproducible Builds effort [0], we noticed that
python-oslo.messaging could not be built reproducibly.

This is because the documentation captures the hostname of the build
system.

[0] https://reproducible-builds.org/

This patch uses sample_default from oslo.config to fix this.

Please accept this patch to fix oslo.messaging, magnum, and zaqar
(at least, probably others).

Change-Id: Ie8717e182f709cbddd645a356789e262b47646d3

oslo_messaging/_drivers/impl_rabbit.py

@@ -246,8 +246,10 @@ rabbit_opts = [
                 default=False,
                 help='Should we use consistant queue names or random ones'),
     cfg.StrOpt('hostname',
+               sample_default='node1.example.com',
                default=socket.gethostname(),
-               help='Hostname used by queue manager'),
+               help='Hostname used by queue manager. Defaults to the value '
+               'returned by socket.gethostname().'),
     cfg.StrOpt('processname',
                default=os.path.basename(sys.argv[0]),
                help='Process name used by queue manager'),