add default "backupper" user password

This password is not really used (ssh connection is key- based), but without a
password, and sshd's `UsePAM` set to `no`, the connection cannot be established
due to the fact that the password field in the shadow file contains "!", which
sshd then intrepids as a "locked account":

  User backupper not allowed because account is locked

If you have a need to know this password, you should override it with your own.
release v0.9.1