Install Composer to PIFR machines

Problem/Motivation

This is a bit of a blocker for #1923582: Add ability for testbot to run 'composer install' during installation

In order to move forward on a bunch of D8 issues related to composer, the testbot needs to be able to install dependencies using composer.

PIFR machines might not have composer installed, so they should.

Proposed resolution

Determine if PIFR machines have composer installed. If so, we're done. Otherwise:

Add a globally-installed composer as a dependency of the PIFR machine builds.

Add a daily/weekly-ish cron call to composer selfupdate.

Remaining tasks

User interface changes

API changes

Data model changes