Install composer for system-wide usage

Install composer by downloading the phar from getcomposer.org and
installing it in /usr/bin. Also monitor the last mod time of composer on
each puppet run and attempt a self-update if composer is more than
2 weeks old.

Change-Id: Icecfc404060795c590a8c21400d4606f0a6e8b96
Merge "Install composer for system-wide usage"