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: Id5c437e98c276a3183b84531749f722705fb8f09

ceilometer/opts.py

@@ -90,6 +90,8 @@ def list_opts():
             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'),
         ]),

releasenotes/notes/bug-1929178-a8243526ce2311f7.yaml

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