Horizon tenant list call should be cached

I'm going to treat this as a request for caching rather than a bug report about the requests in the middleware, since there's already a ticket for that: https://bugs.launchpad.net/horizon/+bug/942074

FWIW, that only makes one request for the tokens (it used to make multiple calls in many cases), however if you're on a page like the syspanel tenants list, the launch instance dialog, the create user dialog, or any other place that makes a separate call to populate a tenant chooser dropdown, etc. then you will see multiple calls and this is correct behavior under the current assumptions in the dashboard.

I've been thinking a lot about the caching problem, though, and it's not simple. We need to cut down on making this tenants call so many times, however we also don't want to find ourselves stuck in a case where a newly-created tenant is inaccessible. This is going to be a tricky task and should be addressed in Folsom.

Right now, as far as I can tell, the authorized_tenant attribute on the request.user object is always set to None, and it should actually be used to cache the tenant list for the logged in user.

The authorized tenants list is set correctly at this point (it was fixed in essex) but there are probably places where it can be put to greater use to reduce extra API calls.

This has slowly gotten better over time. This ticket is now too broad for any remaining work. Specific cases where the cached value could be used to greater effect can be tracked separately.