DCI_ComposerInstall fails under vagrant

Problem/Motivation

Drupal 8.1.x has removed the /vendor directory from the core repo: #1475510: Remove external dependencies from the core repo and let Composer manage the dependencies instead

This means that our normal testing procedure fails because we can't run tests. They fail because they're unable to find vendor/autoload.php.

This means we have to somehow trigger composer to install the dependencies during test runs.

This can be accomplished by setting DCI_ComposerInstall=true #2315537-56: Install composer dependencies before running tests

However, this triggers an composerinstall preprocess step which can't find the composer binary under vagrant.

Proposed resolution

In IRC Mixologic and Mile23 came up with changing DrupalCI\Plugin\BuildSteps\setup\Composer:: buildComposerCommand() to return "./bin/composer $data $workingdir"

This forces the use of whatever Composer binary was added to the testrunner project during its installation, for either the vagrant box or the production server.

Remaining tasks

User interface changes

API changes

Data model changes