nixos/fail2ban: don't use types.string (it's deprecated)

I think neither .daemonConfig nor the .jails options need the ability to
join multiple definitions, hence types.str (instead of types.lines).
nixos/fail2ban: allow overriding the default jails

This allows users to set their own services.fail2ban.jails.DEFAULT
settings (and customizing, or even disabling, the currently
always-enabled ssh filter).