The end-goal here is to be able to do version discovery rather than
relying on config and in-code version defaults for what gets attached to
the connection as the service proxy for a service. However, since we're
currently constructing Adapter objects at instantation time we'd have to
authenticate AND do version discovery on every service when we create a
Connection to be able to do that.
That would be bad.
Add a Descriptor class that creates the Proxy object on-demand. That is,
when someone does "conn.compute", a Proxy will be created and returned.
To support doing that without a ton of duplicate copy-pasta for each
service, add a metaclass to Connection which reads os-service-types and
does the import lookup / BaseProxy fallback as part of Connection class
creation. One of the upsides to this is that we can add docstrings to
the service descriptor objects - meaning that the docs for Connection
actually list the proxy objects.
While we're in here, fix a NOTE in a connection doc string and add a
reference to BaseProxy in the docs so that it'll show up in the docs
too.
Change-Id: I3bef5de60b848146fc8563d853774769d0875c65
ff5b6bc0c9