Use '/' for the vhost if the transport_url has no trailing '/'

This change https://review.openstack.org/#/c/599912/ accidentally
removed setting the virtual host to '/' if none were provided in the
transport_url configuration option.

Closes-Bug: #1798241
Change-Id: I2dbcec3f77eb7cc513350b00ea020cc8faaecec1

oslo_messaging/_drivers/impl_rabbit.py

@@ -595,7 +595,7 @@ class Connection(object):
             parse.quote(host.password or default_password),
             self._parse_url_hostname(host.hostname) or default_hostname,
             str(host.port or 5672),
-            url.virtual_host)
+            url.virtual_host or '')
 
     def _parse_url_hostname(self, hostname):
         """Handles hostname returned from urlparse and checks whether it's