Implement cloned volume for the RBD driver

This adds the support for cloned volume to the RBD driver. This is perform by the
create option --source-volid. Since the clone is pure copy of a volume,
we don't use the clone capability from RBD and simply copy the source
image.

Change-Id: I3b73bc77b74767ffe3583422f56b3603594589ce
Implements: blueprint implement-cloned-volume-for-rbd
Merge "Implement cloned volume for the RBD driver"