Add task for changing a site's installation profile

Coming from #3025393: Imported Platform & Site causes site to not have a Profile set, there are certainly use cases switching sites' installation profiles. For example, a site running on a shared platform with a shared installation profile may with to switch to having its own.

Prerequisites

• The Profile Switcher module, and
• Both the old and new profiles in the /profiles directory.

Switching the profile

We'd just need to run the Profile Switcher's Drush command:

drush @site switch:profile old_profile new_profile

Telling Aegir about it

Here's the SQL that the task should run after Profile Switcher does its thing:

UPDATE hosting_site SET profile = (
  SELECT nid FROM hosting_package WHERE package_type = 'profile' AND short_name = '$PROFILE_NAME'
) WHERE nid = (
  SELECT n.nid FROM node n, (
    SELECT * FROM hosting_site
  ) AS s WHERE n.nid = s.nid AND n.title = '$SITE_NAME'
);

The inner-most subquery is necessary because MySQL doesn't allow you to write queries where the inner query references the one being updated. It's a workaround.