Merge branch '2952999-allow-alter-contexts' of git.drupal.org:project/provision into 2952999-allow-alter-contexts

Issue #2951662: Actually install sudoers.example, to avoid failure on systems ignoring /usr/share/doc/

It turned out that the ubuntu:bionic Dcoker image is ignoring /usr/share/doc/
via /etc/dpkg/dpkg.cfg.d/excludes. Which results in installation failure.

This reverts commit f2e5bf06.

Issue #2941029 by Jon Pugh: Merge remote-tracking branch 'origin/2941029-platform-wide-settings' into 7.x-3.x

I found that when updating MySQL systemd would restart the queued 3 times 100ms apart. While updating MySQL took a bit more then 0.3 seconds.

Herman van Rink authored Jan 31, 2018 and Jon Pugh committed Jan 31, 2018

Issue #2911861 by helmo, Jon Pugh: Hosting Features submit should trigger drush cache clear after verify

Issue #2940702 by Jon Pugh: Unable to override Drupal 8 in local.settings.php: Change `include_once` to `include`. Since Drupal 8 uses `require` and not `require_once`, it's possible settings.php will be included twice. Provision should include custom settings every time.

  - Typo.

  - Cleanup docs and move to provision.api.php