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

setup.cfg

@@ -44,6 +44,7 @@ console_scripts =
     watcher-decision-engine = watcher.cmd.decisionengine:main
     watcher-applier = watcher.cmd.applier:main
     watcher-sync = watcher.cmd.sync:main
+    watcher-status = watcher.cmd.status:main
 
 watcher.database.migration_backend =
     sqlalchemy = watcher.db.sqlalchemy.migration