Revert "disable PermitRootLogin"

This reverts commit 84411a90c85444dcbeee54e5222c34bcc5a76129.
allow UID 0 user (cs-user) to ssh into the machine.
rework on ${INITIAL_USER}

- use variables instead of hardcoded path
- make sure the UID of the user is zero
- set initial password (password authentication is disabled in
sshd_config)