debian: support upstart on Wheezy

Upstart cannot be installed by debootstrap on Wheezy because of a
conflict with sysvinit. sysvinit is flagged a being “essential” and
apt-get will refuse to remove it.
See: https://bugs.debian.org/668001

This patch uses another strategy. We install a standard sysvinit
Debian chroot with debootstrap and during a second step, we
replace sysvinit with the wanted init system.

Change-Id: I081c81b2c32874fccf940c97b79054923c25b8c2
Merge "debian: support upstart on Wheezy"