Only delete key from redis in-memory cache if present (#35126)

Fixes #35120 : the redis cache plugin keeps key/value
entries in an in-memory cache to avoid hitting the
redis database each time.

The problem is that a cache entry is only set when
a value is get or set but it is always deleted when
trying to delete a value.

When the --flush-cache ansible-playbook option is used,
the redis cache plugin is first asked to remove every
entry corresponding to every hostname present in the inventory.
As no value as been set/get so far, it then tries to delete
an unexisting value from the cache and hence crashes with
a KeyError exception.

lib/ansible/plugins/cache/redis.py

@@ -116,6 +116,7 @@ class CacheModule(BaseCacheModule):
         return (self._db.zrank(self._keys_set, key) is not None)
 
     def delete(self, key):
+        if key in self._cache:
             del self._cache[key]
         self._db.delete(self._make_key(key))
         self._db.zrem(self._keys_set, key)