Prototype SSH backends for llab authentication

- Add openssh_copyid_backend for lock-file based
  cat'ing of the ~/.ssh/authorized_keys. This is
  a fall-back method to the preferred way.
- Add llab-authorize-keys to act as an
  authorized_keys_script for supported OpenSSH
  platforms. Your OpenSSH instance must be patched
  to support execution of an authorized_keys_script
  file.
- Add settings.py SSH_BACKEND to allow user to
  choose a backend.
- Add lockfile to requirements.txt
- Simplified IV for encrypt/decrypt