Ability to pass metadata during snapshot create

Metadata as a parameter is supported by the underlying cinder driver.
This is not exposed by the python client. It is helpful to store additional information
for snapshots and can be used for decision within cinder driver during snapshot creation.
Similar functionality exists for volume creation in cinder client and cinder driver.
Updating cli(shell) to accomodate the new metadata parameter
Adding above change for v2

Change-Id: I002221efea1b9994da4a8d200544a88b0ed26277
Closes-Bug: #1341424
Merge "Ability to pass metadata during snapshot create"