update keystone.conf to Icehouse

- Added a lot of `oslo.messaging` attributes which are unused around
 `amqp`, `rabbitmq`, `kombu`. `rpc_backend` now defaults to rabbitmq instead
 of kombu (but we were never changing this value)

- [auth].methods changed default from `external,password,token,oauth1`
 to `external,password,token`. This was set explicitly in our config,
 but I think it was only because that's how it used to be in Havana as
 we weren't using (or configuring) oauth1.

- A lot of the config options which we were hardcoding are now
 defaults (and commented in the sample config). These were left
 commented like upstream:
 * [auth].password/token/oauth1
 * [catalog].driver
 * [ec2].driver
 * [paste_deploy].config_file
 * [policy].driver