Change challenge solutions to use `sshpass`

[teodutu]

Some challenges, especially those from the first 2 sessions have the students SSH into a container and run commands there. The reference solutions for those activities simply contain the commands that need to be run on the server and assume the fact that the student has already established an SSH connection.

This is incomplete because solutions should be able to get the flag when run on any machine. Thefore, the aforementioned solutions need to be updated to use sshpass to forward the required commands to the machines that host each challenge.