Created the volume_type attribute.

From our [support site](http://support.rightscale.com/12-Guides/RightScale_Cloud_Appliance_for_vSphere_(RCA-V)%3A_Installation_Guide/Download,_Deploy,_and_Configure_the_RightScale_Cloud_Appliance_for_vSphere_(RCA-V)#Volume_Types_(v1.1_or_later)), this is a RightScale construct.  I'll update this to match what the is in the support example - "Platinum-Volume-Type".