Use keystoneclient library for all auth 2.0 use

We are currently implementing auth 2.0 in swiftclient to talk to auth 2.0 servers. We could use instead python-keystoneclient without having to reimplementing all of it ourselves. We would have a new dependence on the library only when using auth 2.0. We would need to make sure to handle all the old behavior (i.e: tenant:user with -U )