[openstack|volume] support imageRef option

Volumes can be created with the imageRef option, which dictates which
image to base the volume off.
[openstack|compute] support block_device_mapping

Servers can be booted with block_device_mapping instead of image_id.
[Openstack|volume] Add #get to volumes collection

This fixes reloading Volume objects and makes it consistent with other
collections.

Keeping find_by_id as an alias for compatibility.
[Openstack|volume] fix key name for volume_type

The cinder API uses volume_type instead of type.
[Openstack|volume] create volumes from other vol.
[Openstack|Volumes] alias type to volume_type

This makes the changes consistent with how it previously worked.
Merge pull request #1922 from grk/volume-create-image-ref

OpenStack: create volumes from images, boot servers with block device mapping