Remove XML description from extension concept
Remove XML description from extension concept

XML API interface has been already removed from Nova completely,
and its description of the extension concept becomes meaningless now.
So this patch removes it.

A small amount of the JSON description is left. The details are
trimmed down, as we're actually trying to get people away from
extensions as a concept.

A big warning to that effect is added into the document.

Partially implements blueprint complete-todo-in-api-concept-doc

Change-Id: Icf8bf87f38733ba03cf5ce1225594b0b282fde56

hanlind/nova,phenoxim/nova,cloudbase/nova,dims/nova,rajalokan/nova,BeyondTheClouds/nova,cernops/nova,rajalokan/nova,sebrandon1/nova,jianghuaw/nova,mikalstill/nova,sebrandon1/nova,klmitch/nova,cloudbase/nova,mikalstill/nova,klmitch/nova,alaski/nova,mahak/nova,zhimin711/nova,openstack/nova,rahulunair/nova,rajalokan/nova,BeyondTheClouds/nova,BeyondTheClouds/nova,CEG-FYP-OpenStack/scheduler,NeCTAR-RC/nova,jianghuaw/nova,cyx1231st/nova,Juniper/nova,cernops/nova,vmturbo/nova,bigswitch/nova,cloudbase/nova,NeCTAR-RC/nova,gooddata/openstack-nova,openstack/nova,vmturbo/nova,rahulunair/nova,cernops/nova,klmitch/nova,dims/nova,gooddata/openstack-nova,hanlind/nova,sebrandon1/nova,jianghuaw/nova,openstack/nova,phenoxim/nova,jianghuaw/nova,vmturbo/nova,zhimin711/nova,vmturbo/nova,bigswitch/nova,mahak/nova,Juniper/nova,gooddata/openstack-nova,gooddata/openstack-nova,mikalstill/nova,CEG-FYP-OpenStack/scheduler,Juniper/nova,Juniper/nova,alaski/nova,rahulunair/nova,hanlind/nova,rajalokan/nova,cyx1231st/nova,mahak/nova,klmitch/nova