Explicitly declare Train supported runtimes.

The TC added openSUSE Leap as a PTI OS in the Stein cycle, and at a
TC ad-hoc meeting [1] the TC defined python 3.6 and 3.7 as the required
versions for the Train cycle.

Golang is still in a rapid development state, and as a result we do not have
a recommended stable golang version for all projects.

1 - http://eavesdrop.openstack.org/meetings/tc_python3/2019/tc_python3.2019-03-07-21.00.html

Change-Id: Ibf0c77f409fb83ac20ddaac4a96f21f7d6516165
Signed-off-by: Graham Hayes <gr@ham.ie>

reference/runtimes/train.rst

@@ -0,0 +1,35 @@
+=========================
+Tested Runtimes for Train
+=========================
+
+It is the policy that each OpenStack release cycle will target the
+currently available runtimes on the :ref:`LTS or stable
+distributions <pti-linux-distros>` at the start of a development cycle.
+
+At the start of the Train development cycle, the current versions are:
+
+* Ubuntu 18.04
+* CentOS 7
+* openSUSE Leap 15
+
+Python Runtime for Train
+========================
+
+Based on the available Python runtimes for the supported Linux distributions,
+all Python-based projects must target and test against, at a minimum:
+
+* Python 2.7
+* Python 3.6
+* Python 3.7
+
+More details on Python requirements can be found in :ref:`pti-python`.
+
+Golang Runtime for Train
+========================
+
+At this time, there are still frequent releases of Go with a wide variety of
+distribution packaged versions. Given the current state of Go support and the
+number of projects within OpenStack using Go, no formal version declaration is
+being made at this time.
+
+More details on Go requirements can be found in :ref:`pti-golang`.