Allow the user to update a volume's metadata

Allow the user to update a volume's metadata

The fix mainly involves updating the VolumeController's update method
to allow the 'metadata' key to be a valid key in a request body.

The _translate_volume_summary_view method is updated to check for a
'metadata' key in the vol parameter after checking first for
'volume_metadata'.  This is to avoid making a third call to the
database after updating the volume model.  This solution introduces
the least amount of new code.

Change-Id: I41256d03f9c4c7a0866ff0f2d35fce8e4fd26b11
Fixes: bug #1046382