Implement methods to modify volume metadata.

There are four methods in nova/volume/cinder.py which are
NotImplemented. They are as follows:
  1. get_volume_metadata
  2. delete_volume_metadata
  3. update_volume_metadata
  4. get_volume_metadata_value
These methods are required in cases where nova needs to modify
a cinder volume's metadata, e.g. attach and detach time. It also
completes the cinder.API class.

Change-Id: I360076ae28db43e661466f556425947813c9040e
Closes-Bug: #1296164
Merge "Implement methods to modify volume metadata."