Add TransportURL unparse function

Currently, the TransportURL class has a classmethod 'parse' that creates a
TransportURL object from a transport url string. We have a use case where
we need to extract a transport url string from a ConfigOpts object.
In Nova Cells v2, we have some commands operators will run to set up new
cells. Each new cell record has a transport_url string to allow the parent
cell to connect to child cell message queue. The operator provides a nova.conf
config file to the command and the config object contains the transport url
scattered among various config options. We would like to be able to call an
oslo.messaging function to get the transport url as a string from the
config object to use in the cell record.