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.
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