Add "Train" to glossary

Add new release "train" to glossary.

Explanation taken from
http://lists.openstack.org/pipermail/openstack-dev/2018-October/135837.html

Change-Id: If24a1bf67bd1ab7ad3291ab33a749b18ca5be52f
Co-Authored-By: Jay S Bryant <jungleboyj@gmail.com>

doc/common/glossary.rst

@@ -2754,7 +2754,8 @@ O
       Each OpenStack release has a code name. Code names ascend in
       alphabetical order: Austin, Bexar, Cactus, Diablo, Essex,
       Folsom, Grizzly, Havana, Icehouse, Juno, Kilo, Liberty,
-      Mitaka, Newton, Ocata, Pike, Queens, Rocky, and Stein.
+      Mitaka, Newton, Ocata, Pike, Queens, Rocky, Stein,
+      and Train.
       Code names are cities or counties near where the
       corresponding OpenStack design summit took place. An
       exception, called the Waldon exception, is granted to
@@ -3807,6 +3808,22 @@ T
       Community project used to run automated tests against the
       OpenStack API.
 
+   Train
+
+      The code name for the twentieth release of OpenStack. The
+      OpenStack Infrastructure Summit will take place in Denver,
+      Colorado, US.
+
+      Two Project Team Gathering meetings in Denver were held at a
+      hotel next to the train line from downtown to the airport. The
+      crossing signals there had some sort of malfunction in the past
+      causing them to not stop the cars when a train was coming
+      properly. As a result the trains were required to blow their
+      horns when passing through that area. Obviously staying in a
+      hotel, by trains that are blowing their horns 24/7 was less than
+      ideal. As a result, many jokes popped up about Denver and
+      trains - and thus the release is called train.
+
    transaction ID
 
       Unique ID assigned to each Object Storage request; used for