[W.I.P] Make Rackspace drivers multi-datacenter aware 

This branch does the following:

* Updates all the Rackspace drivers to use auth 2.0
* Updates all the Rackspace drivers to always use global auth endpoint
* Updates Rackspace compute driver to correctly handle `datacenter` kwarg with next gen driver and `region` kwarg with first gen driver
* Updates Rackspace storage driver to correctly handle `datacenter` kwarg

Still left to do and update:

* Update affected compute tests
* Update Loadbalancer drivers
* Update DNS drivers
* Eventually drivers which support that (every driver except first gen Cloud Servers driver) should be updated to user service catalog instead of hard-coded list of available datacenters