Add note for running designate-tempest-plugins
Add note for running designate-tempest-plugins

It's not trivial to run tempest plugin tests if you don't
set things up exactly right, this explains an annoying
nuance to make it easier.

Change-Id: I759cb747504cf47548ae4460ffa67ba62c49024b

openstack/designate,openstack/designate,openstack/designate