Rackspace block storage

This PR adds support for Rackspace's implementation of Cinder; covers #218 task
You should do a `opts := tools.OnlyRS(opts)` here.

`rackspace.AuthOptionsFromEnv()` will fall back to `OS_*` env vars if `RS_` ones are missing, so that we work out of the box with the same credentials that tools like Supernova use. But, in the acceptance tests, since we have both sets of credentials at once, they collide in awkward ways; for example, the presence of `OS_AUTH_URL` will make the Rackspace tests run against devstack! `tools.OnlyRS()` strips out the `OS_*` credentials for you and keeps them cleanly separated.