Use eventlet executor in rpc_service

The default executor of o.m is 'blocking', but after change [1],
o.m with blocking is broken. 'eventlet' and 'non-blocking' work
fine and most services use 'eventlet', so this is why it was
noticed by magnum which used the default one.

Use eventlet to not be affected by bug #1694728 , but more
importantly, the oslo team suggests to not use blocking which
has 0% test coverage and it is going to be deprecated and unset
from the default configuration.

[1] https://review.openstack.org/#/c/463673/

Change-Id: I47da73787456c97f7d84fd4440404b551ff62528
Closes-Bug: #1694728

magnum/common/rpc_service.py

@@ -50,6 +50,7 @@ class Service(service.Service):
         # TODO(asalkeld) add support for version='x.y'
         target = messaging.Target(topic=topic, server=server)
         self._server = messaging.get_rpc_server(transport, target, handlers,
+                                                executor='eventlet',
                                                 serializer=serializer)
         self.binary = binary
         profiler.setup(binary, CONF.host)