Deprecate keystone.conf.memcache socket_timeout

Change-Id: I5de14b5bd2d96c2f78152eda48842d388109e02b
Partial-Bug: #1838037
Closes-Bug: #1837407

keystone/conf/memcache.py

@@ -11,6 +11,7 @@
 # under the License.
 
 from oslo_config import cfg
+from oslo_log import versionutils
 
 from keystone.conf import utils
 
@@ -26,6 +27,11 @@ This is used by the key value store system.
 socket_timeout = cfg.IntOpt(
     'socket_timeout',
     default=3,
+    deprecated_for_removal=True,
+    deprecated_reason='This option is duplicated with oslo.cache. '
+                      'Configure ``keystone.conf [cache] memcache_socket_timeout`` '
+                      'option to set the socket_timeout of memcached instead. ',
+    deprecated_since=versionutils.deprecated.TRAIN,
     help=utils.fmt("""
 Timeout in seconds for every call to a server. This is used by the key value
 store system.

releasenotes/notes/deprecated-socket_timeout-option-d3358b4f2310706c.yaml

@@ -0,0 +1,12 @@
+---
+deprecations:
+  - |
+    The socket timeout configuration option for memcache of Keystone's
+    definition isn't actually used anywhere [0], it would appear to be
+    a broken knob. In fact oslo.cache has a duplicate option that appears
+    to be used instead [1].
+    We can deprecate the keystone-specific option and point people to the
+    oslo.cache option.
+
+    [0] https://opendev.org/openstack/keystone/src/commit/a0aa21c237f7b42077fc945f157844deb77be5ef/keystone/conf/memcache.py#L26-L32
+    [1] https://opendev.org/openstack/oslo.cache/src/commit/a5023ba2754dd537c802d4a59290ff6378bd6285/oslo_cache/_opts.py#L85-L89