Three changes:
* Add set_default_selinux_context() to module_common that sets
  a file's context according to the defaults in the policy
* In atomic_replace(), set the default context for the file if
  selinux is enabled and the destination file does not exist.
* In authorized_key, set the default context when creating
  $HOME/.ssh and $HOME/.ssh/authorized_keys.  If these already
  exist, this won't touch them.