Add a Firewall & fail2ban & SSH key-based authentication

[motters]

Implement a basic firewall using iptables which blocks all ports except ssh and outgoing http/s ports. Maybe check issues with the firewall and mendor.

Also implement fail2ban to block IP after multiple failed attempts.

Locking down ssh to a key-based authentication is also a necessary.

[motters]

Basic firewall has be implemented using iptables which only allows inbound SSH connections.

Not yet implemented key-based authentication.