Added flag to force a pseudo-tty allocation  (-y or --force-tty)

In order to avoid an error like 'pseudo-terminal will not be allocated because stdin is not a terminal' due to the fact that when Nagios runs the check_by_ssh plugin it doesn't have a TTY, we need to be able to tell the SSH command to force a pseudo-tty allocation. Hence, the --force-tty (or -y) flag.