api-ref: add note about os-certificates API
api-ref: add note about os-certificates API

The os-certificates API is weird, and only wasn't removed during the
ec2 drop because we exposed it in the osapi. It will be dropped in the
future, and we should tell people to not use it starting now.

Change-Id: I95f8a9620f7eb865298b656477f77257de91b2cc

Juniper/nova,phenoxim/nova,cloudbase/nova,Juniper/nova,mahak/nova,rajalokan/nova,sebrandon1/nova,vmturbo/nova,hanlind/nova,cloudbase/nova,mikalstill/nova,openstack/nova,alaski/nova,phenoxim/nova,jianghuaw/nova,rahulunair/nova,mahak/nova,sebrandon1/nova,vmturbo/nova,jianghuaw/nova,openstack/nova,openstack/nova,jianghuaw/nova,mikalstill/nova,rajalokan/nova,Juniper/nova,vmturbo/nova,gooddata/openstack-nova,gooddata/openstack-nova,mikalstill/nova,rajalokan/nova,rajalokan/nova,gooddata/openstack-nova,hanlind/nova,hanlind/nova,klmitch/nova,mahak/nova,cloudbase/nova,rahulunair/nova,sebrandon1/nova,gooddata/openstack-nova,klmitch/nova,Juniper/nova,jianghuaw/nova,alaski/nova,rahulunair/nova,klmitch/nova,klmitch/nova,vmturbo/nova