Allow to enable default Nginx filters

Just need a variable in defaults, can be false by default.

root@autonomic-swarm:~# ls /etc/fail2ban/filter.d/ | grep -i nginx
nginx-botsearch.conf
nginx-http-auth.conf
nginx-limit-req.conf

mentioned in commit d80eef1a

mentioned in merge request !3 (merged)

mentioned in commit 2a59177b

closed via merge request !3 (merged)

mentioned in commit 7204d98e