Add support for defining attributes from environment variables

[terojankkila]

In container world we don't always have easy access to installer script and it's parameters but we do have easy access to defining new environment variables.

So I would humbly request a feature that would allow me to define something like COMPOSER_CHANNEL=1 which would be same as giving --1 for the installer script. And to keep logic clean giving --1 should override environment variables.

Having all variables available as environment variables would ease use of composer installer in docker/Openshift containers.

br, Tero Jankkila

[Seldaek]

If you like to send a PR for the installer script (+ tweak the download page with docs perhaps) that sounds fine to me.

[MT4T]

default['mycookbook']['myvar'] = '3'