- Cleanup docs and move to provision.api.php

  - If check for shell_exec fails, log a warning.

Merge branch '2937147-composer-install' of git.drupal.org:project/provision into 2937147-composer-install

 - Wrap the whole thing in a check for a drush option `provision_composer_install_platforms`. If set to FALSE, don't even check for composer.json.
 - Make `provision_composer_install_command` a variable, allowing customization.
 - Rename the variable `provision_composer_install_platforms`.

This reverts commit 76ff4485.

Issue #2884608 by helmo, josebc, MartijnBraam: Site language not set when provisioning a drupal 8 site

Combination of manual and except was not working ... rename a branch to have the -runalltests suffix to get all tests done.

Add lint check and set some tests to manual that were for a master branch only.