Clearly identify boot2docker in /etc/os-release
Clearly identify boot2docker in /etc/os-release

Signed-off-by: Ben Firshman <73675debcd8a436be48ec22211dcf44fe0df0a64@firshman.co.uk>

bsmr-docker/boot2docker,skippbox/boot2docker,docker/boot2docker,tomgco/boot2docker,pdevine/boot2docker