move default_volume_type to cinder::api

In a previous commit, this parameter has been introduced in
cinder::backends, but it should be configured on Cinder API server.
This patch deprecated the old parameter.

Change-Id: Ic84c316dc80d8d778ce62df85a6b5bff4a18ead1
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>