the value in `docker_exec_client_cmd` doesn't allow to check for
existing pools because it's set with a wrong value for the entrypoint
that is going to be used.
It means the check were going to fail anyway even if pools actually exist.

Using jinja syntax to set `docker_exec_cmd` allows to handle the case
where you don't have monitors in your inventory.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>