Deprecate unused [coordination] check_watchers

The [coordination] check_watchers is no longer used in any
implementation. This change deprecates the parameter so that we can
remove it in a future release.

Closes-Bug: #1929178
Change-Id: I973eb15433e66c75366666940a841e5a579bf0e8

aodh/coordination.py

@@ -38,6 +38,8 @@ OPTS = [
                       'coordination.'),
     cfg.FloatOpt('check_watchers',
                  default=10.0,
+                 deprecated_for_removal=True,
+                 deprecated_reason='This parameter is no longer used.',
                  help='Number of seconds between checks to see if group '
                       'membership has changed'),
     cfg.IntOpt('retry_backoff',

releasenotes/notes/bug-1929178-46493335946174a5.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    The ``[coordination] check_watchers`` parameter has been deprecated since
+    it has been ineffective.