Allow list_extensions to work in cinderclient v2

Cinder client extensions also requires explicity setting the
service_type that they are available for. This makes the only extension
for the client available to v2 as well.

Change-Id: I9a73cac31c70b248ac6070540554f9293e9a6e43
Closes-Bug: #1284321
Merge "Allow list_extensions to work in cinderclient v2"