Add framework for watcher-status upgrade check

This commit adds the functionality of watcher-status CLI for performing
upgrade checks as part of the Stein cycle upgrade-checkers goal.
It only includes a sample check which must be replaced by real checks in
future.

Change-Id: Ic3d066af439797d6f705e805334f729b52ce3aac
Story: 2003657
Task: 26164

doc/source/man/index.rst

@@ -5,4 +5,5 @@
    watcher-api
    watcher-applier
    watcher-db-manage
-   watcher-decision-engine
+   watcher-decision-engine
+   watcher-status

doc/source/man/watcher-status.rst

@@ -0,0 +1,83 @@
+==============
+watcher-status
+==============
+
+-----------------------------------------
+CLI interface for Watcher status commands
+-----------------------------------------
+
+Synopsis
+========
+
+::
+
+  watcher-status <category> <command> [<args>]
+
+Description
+===========
+
+:program:`watcher-status` is a tool that provides routines for checking the
+status of a Watcher deployment.
+
+Options
+=======
+
+The standard pattern for executing a :program:`watcher-status` command is::
+
+    watcher-status <category> <command> [<args>]
+
+Run without arguments to see a list of available command categories::
+
+    watcher-status
+
+Categories are:
+
+* ``upgrade``
+
+Detailed descriptions are below:
+
+You can also run with a category argument such as ``upgrade`` to see a list of
+all commands in that category::
+
+    watcher-status upgrade
+
+These sections describe the available categories and arguments for
+:program:`Watcher-status`.
+
+Upgrade
+~~~~~~~
+
+.. _watcher-status-checks:
+
+``watcher-status upgrade check``
+  Performs a release-specific readiness check before restarting services with
+  new code. For example, missing or changed configuration options,
+  incompatible object states, or other conditions that could lead to
+  failures while upgrading.
+
+  **Return Codes**
+
+  .. list-table::
+     :widths: 20 80
+     :header-rows: 1
+
+     * - Return code
+       - Description
+     * - 0
+       - All upgrade readiness checks passed successfully and there is nothing
+         to do.
+     * - 1
+       - At least one check encountered an issue and requires further
+         investigation. This is considered a warning but the upgrade may be OK.
+     * - 2
+       - There was an upgrade status check failure that needs to be
+         investigated. This should be considered something that stops an
+         upgrade.
+     * - 255
+       - An unexpected error occurred.
+
+  **History of Checks**
+
+  **1.12.0 (Stein)**
+
+  * Sample check to be filled in with checks as they are added in Stein.