Allow any hosting task to flag itself for needing a provision-save

In modules/hosting/task.hosting.inc, function drush_hosting_task() hard codes which Task Types must call provision-save before running the primary task. Only task types "install", "verify", and "import" do this. I have a task in devshop_projects.module that creates a new "context" type in Aegir, but it is blocked by this hard coding.

Current Code From: drush_hosting_task(), modules/hosting/task.hosting.inc:

// On install/verify, save the named context
  if ($task->task_type === 'install' || $task->task_type === 'verify' || $task->task_type === 'import') {
    //...
    $output = drush_invoke_process('@none', 'provision-save', array('@' . $task->ref->hosting_name), $task->context_options, array('method' => $mode));
  }

This patch changes the above to:

// On install/verify, save the named context
  if (!empty($task->task_info['provision_save'])) {
    //...
    $output = drush_invoke_process('@none', 'provision-save', array('@' . $task->ref->hosting_name), $task->context_options, array('method' => $mode));
  }

The following patch changes the logic behind determining what tasks to call provision-save for. Instead of a hard coded fixed list, hook_hosting_tasks() is used to set a $task->provision_save boolean whether or not the task requires this.

Patched version of 'install' task from hosting_site_hosting_tasks():

$tasks['site']['install'] = array(
    'title' => t('Install'),
    'description' => t('Install a site'),
    'hidden' => TRUE,
    'provision_save' => TRUE,
  );

I'd love some feedback to see if this is the right direction, then I will patch against 1.x and 2.x