package/fail2ban: add fixup for default configuration

fail2ban default configuration is not compatible with buildroot as is.
In order to not force the user to overwrite it in an overlay and
having a fail2ban service running out of the box, this patch adds a
fixup step to adapt the default configuration to running on buildroot.

The dbfile is None to not add a dependency on
BR2_PACKAGE_PYTHON_SQLITE.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>