Automatic merge from submit-queue

Rackspace improvements (OpenStack Cinder)

This adds PV support via Cinder on Rackspace clusters. Rackspace Cloud Block Storage is pretty much vanilla OpenStack Cinder, so there is no need for a separate Volume Plugin. Instead I refactored the Cinder/OpenStack interaction a bit (by introducing a CinderProvider Interface and moving the device path detection logic to the OpenStack part).

Right now this is limited to `AttachDisk` and `DetachDisk`. Creation and deletion of Block Storage is not in scope of this PR.

Also the `ExternalID` and `InstanceID` cloud provider methods have been implemented for Rackspace.