Discover AUTH_URL from DNS

A feature that has been requested for a long time is being able to discover the AUTH_URL of a plugin from DNS rather than have to set it in your ENV or on the command line. This had been a bit of a problem with keystoneclient because of the same object being used to load plugins as well as be the plugin, and this is a loading problem.

Now we've split those two concepts you should be able to define a loader that can find an auth_url from a DNS TXT record if one isn't provided. It should not be required that you have to add additional flags to get discovery, simply ommitting a hardcoded auth_url is enough.

This auth_url should be a pointer to the root discovery URL of keystone and therefore only needs to work with the Generic series of plugins that are capable of doing version discovery from this base URL.