MDLSITE-2885 Install composer into alt directory

This script will automatically install and update
the composer stuff as specified by Moodle's composer.json
keeping it out from the dirroot/vendor and installing it
into another alternative path, to avoid it being cleaned
for every build.

Any other script requiring composer stuff (phpunit, behat for now)
will have to execute this script and, magically, the executables
will be available for them.
MDLSITE-2885 Run phpunit form composer.