[das-toolbox] Enhance remote servers validation on das-cli config and remote commands in general

[levisingularity]

Currently, during the remote server setup in das-cli, we validate whether an SSH connection to the provided IP address is reachable. However, this validation does not enforce any restrictions on the use of the root user for remote connections. For security reasons, the validator should be improved to ensure that connections are established only with valid IP addresses and that attempts to connect using the root user are explicitly disallowed.