Add four new options and update README

  - Add support for SSSD. There are
    now two boolean values that support
    disabling and enabling SSSD and SSSD Auth.
    These are passed in as standalone flags.
  - Add support for PAM access via a boolean
    variable. This is passed in via 'extra flags'.
  - Add support for FORCELEGACY via a boolean variable.
    This is passed in via 'extra flags'
  - README includes descriptions of options
    functionality.