With central attachdetach controller, we don't need to lock the image
any more. But for backward compatibility, we should:
1) Check if the image is still used by nodes running old kubelet in
attaching.
2) Clean old rbd.json file and remove lock if found in detaching.
1) Modify rbdPlugin to implement volume.AttachableVolumePlugin
interface.
2) Add rbdAttacher/rbdDetacher structs to implement
volume.Attacher/Detacher interfaces.
3) Add mount.SafeFormatAndMount/mount.Exec fields to rbdPlugin, and
setup them in rbdPlugin.Init for later uses.
Attacher/Mounter/Unmounter/Detacher reference rbdPlugin to use mounter
and exec. This simplifies code.
4) Add testcase struct to abstract RBD Plugin test case, etc.
5) Add newRBD constructor to unify rbd struct initialization.
