docs: add concept guide for certificate
docs: add concept guide for certificate

This patch adds the concept guide for certificate.

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

Change-Id: If44e5f4633ee47d235e8254904d950e7d81d39c1

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