Switch from '--invoke' to '--strict=0' for backend calls

It looks like just switch from 'invoke' => TRUE to 'strict' => FALSE in the 3 places we do this should be sufficient. See attached patch.

Actually, I got this error when attempting to install a site:

Unknown option: --client_email. See `drush help provision-install-backend` for available options. To suppress this error, add the option --strict=0.

So, we can either add 'client_email' as an option to 'provision-install-backend' (which I think is the correct solution):

  $items['provision-install-backend'] = array(
    'description' => dt('Provision a new site using the provided data.'),
    'options' => array(
      'client_email' => dt('The email address of the client to use.'),
    ),
    'hidden' => TRUE,
    'bootstrap' => DRUSH_BOOTSTRAP_DRUPAL_SITE
  );

Or we could call provision_backend_invoke() with --strict=0:

$result = provision_backend_invoke(d()->name, "provision-install-backend", array(), array('client_email' => drush_get_option('client_email'), 'strict' => FALSE));

Yes, it's compatible with Drush 5.

Fixed in f488709 for Hosting, and c8febbc in Provision.

no, really, fixed!