Add a TransportURL class to the public API

Nova's cells/rpc_driver.py has some code which allows user of the REST
API to update elements of a cell's transport URL (say, the host name of
the message broker) stored in the database. To achieve this, it has
a parse_transport_url() method which breaks the URL into its constituent
parts and an unparse_transport_url() which re-forms it again after
updating some of its parts.

This is all fine and, since it's fairly specialized, it wouldn't be a
big deal to leave this code in Nova for now ... except the unparse
method looks at CONF.rpc_backend to know what scheme to use in the
returned URL if now backend was specified.

oslo.messaging registers the rpc_backend option, but the ability to
reference any option registered by the library should not be relied upon
by users of the library. Imagine, for instance, if we renamed the option
in future (with backwards compat for old configurations), then this
would mean API breakage.

So, long story short - an API along these lines makes some sense, but
especially since not having it would mean we'd need to add some way to
query the name of the transport driver.

In this commit, we add a simple new TransportURL class:

  >>> url = messaging.TransportURL.parse(cfg.CONF, 'foo:///')
  >>> str(url), url
  ('foo:///', <TransportURL transport='foo'>)
  >>> url.hosts.append(messaging.TransportHost(hostname='localhost'))
  >>> str(url), url
  ('foo://localhost/', <TransportURL transport='foo', hosts=[<TransportHost hostname='localhost'>]>)
  >>> url.transport = None
  >>> str(url), url
  ('kombu://localhost/', <TransportURL transport='kombu', hosts=[<TransportHost hostname='localhost'>]>)
  >>> cfg.CONF.set_override('rpc_backend', 'bar')
  >>> str(url), url
  ('bar://localhost/', <TransportURL transport='bar', hosts=[<TransportHost hostname='localhost'>]>)

The TransportURL.parse() method equates to parse_transport_url() and
TransportURL.__str__() equates to unparse_transport().

The transport drivers are also updated to take a TransportURL as a
required argument, which simplifies the handling of transport URLs in
the drivers.

Change-Id: Ic04173476329858e4a2c2d2707e9d4aeb212d127