Document Cinder Driver Base Features

To avoid having a grid of what features are available by which drivers and which releases, the Cinder team has met and agreed on 2013-04-24 that we would request all current and new drivers to fulfill a list of minimum requirement features [1] in order to be included in new releases.

To make things easier for new/current drivers, it would be great to have this information accurate and accessible in the dev docs.

[1] - https://wiki.openstack.org/wiki/Cinder#Minimum_Driver_Features