Add capability to update volume metadata.

This addresses some cleanup and bugs with volume metadata updates
on the cinder side.  Mostly this implements v1/volume_metadata api
and adds things like propery checks to cinder.volume.api.update_volume_meta.

NOTE: This is only for api V1, a seperate patch will be provided for
V2 once this lands.

The remainder of the bp will be implemented in a cinderclient patch to follow.
Implement cinder side of blueprint update-vol-metadata

Also fixes bug: 1096018

Change-Id: Ie11931f657d3dcd69680fe5fcae435ff89549d97
Merge "Add capability to update volume metadata."