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.
ee3dfef016