The neutron standard-attr-description is not working with NSX
plugins for security group rules. It seems that when the extension
is loaded the relevant DB model class is not yet available.
To address this problem, this change explictly adds a resource
extender function for the NSX plugin to add the description field
to ecurity groups rule responses.
Change-Id: I4d8b2629660f9e33401ce6b011b1784a6ec66aac