Add command to show all service versions

Knowing what services and what versions of those services exist on a
cloud isn't always a spectacular experience. Add a command that will use
get_all_version_data from keystoneauth to produce a report of the
available services and the version info for each service.

Depends-On: https://review.openstack.org/584944
Change-Id: I84751c175d0c5f6d857a5473d2db6d5f1b41f946

doc/source/cli/command-objects/versions.rst

@@ -0,0 +1,41 @@
+========
+versions
+========
+
+Get a list of every version of every service in a given cloud.
+
+versions show
+-------------
+
+Show service versions:
+
+.. program:: versions show
+.. code:: bash
+
+    openstack versions show
+        [--all-interfaces]
+        [--interface <interface>]
+        [--region-name <region-name>]
+        [--service <service>]
+
+.. option:: --all-interfaces
+
+    Return results for every interface of every service.
+    [Mutually exclusive with --interface]
+
+.. option:: --interface <interface>
+
+    Limit results to only those on given interface.
+    [Default 'public'. Mutually exclusive with --all-interfaces]
+
+.. option:: --region-name <region-name>
+
+    Limit results to only those from region-name
+
+.. option:: --service <service>
+
+    Limit results to only those for service. The argument should be either
+    an exact match to what is in the catalog or a known official value or
+    alias from `service-types-authority`_.
+
+.. _service-types-authority: https://service-types.openstack.org/