roles/common: use pyinotify backend for nginx fail2ban jail

This seems to be automatically selected, but on some other servers
I notice it is not. I will set it here explicitly so fail2ban does
not fall back to the inefficient "polling" or incorrect "systemd"
backends.

roles/common/templates/etc/fail2ban/jail.d/nginx.local.j2

@@ -9,6 +9,7 @@ banaction=nftables[type=allports]
 # Integrate with firewalld and ipsets
 banaction = firewallcmd-ipset
 {% endif %}
+backend   = pyinotify
 logpath   = /var/log/nginx/*-access.log
 # Try to find a non-existent wp-login.php once and get banned. Tough luck.
 maxretry  = 1