Update contrib/mkimage/debootstrap to account for Debian LTS changes
Update contrib/mkimage/debootstrap to account for Debian LTS changes

See https://www.debian.org/News/2016/20160425 and https://wiki.debian.org/LTS/Using for more details.

> For Debian 7 "Wheezy" LTS there will be no requirement to add a separate wheezy-lts suite to your sources.list any more. In fact you will not notice the switch to LTS because after the official security support by the Debian Security Team ends, security updates will be provided via security.debian.org 's wheezy/updates.

Signed-off-by: Andrew "Tianon" Page <4b2b69742da531f1900c4c4748d7a0ecda59626a@gmail.com>

TheBigBear/docker,TheBigBear/docker,TheBigBear/docker