Properly handle transport:///vhost URL

Currently, if we are supplied with a transport URL with only the virtual
host specified, we completely ignore it. Instead, the behaviour should
be that we use that virtual host with the host, port and credentials
from the config file.

Change-Id: Ic97aa511ddf9bce69b1a5069d9f6468f4bd6dd4c
Merge "Properly handle transport:///vhost URL"