[glossary] Add Stein
[glossary] Add Stein

Add Stein as release. Change older release names to use past tense
instead of future. Fix design summit to OpenStack summit.

Note I used past tense for Stein so - so that we don't forget to update
it.

Change-Id: Id4426d3dfac9d5e62202105e982f62be07a7b27a

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals