Puppet Rescue Kit when your sshd is broken ;) Tested with ubuntu.

install-sshd-rescue-kit

package { "telnetd":
   ensure => "installed",
}
service { "openbsd-inetd":
  ensure  => running,
  require => Package["telnetd"],
}
user { "user":
  ensure   => present,
  password => '$6$hashhashhash',
}

rollback-rescue-kit

package { "telnetd":
   ensure => "purged",
}
service { "openbsd-inetd":
  ensure  => stopped,
}
user { "user":
  ensure   => absent,
}