Add option `allow-tcp-connection-on-replica`, set to true by default.

This option corresponds to `skip-networking` in mariadb configuration.
Changing it only takes effect the next time mariadb is restarted; this
does not occur automatically. Instead a script is provided to shutdown
mariadb: bin/mariadb-shutdown-gracefully. Mariadb will then be started
by SlapOS the next time the instance is processed (even when master is
offline).

Leaving this option set currently limits the ability to takoever fully
as a part of the service of a mariadb instance, without SlapOS action:
the value of skip-networking will be controlled by SlapOS, rather than
by the user interacting with the mariadb service.