Added INSTALL_DEBIAN_INIT_SCRIPT test.

This checks for the INSTALL_DEBIAN_INIT_SCRIPT environment variable and
will skip trying to install upstart.  This is a method of installing the
salt-minion init script on an Ubuntu LXC because upstart wasn't working.
The method for bootstrapping salt in the LXC is like so:

```
lxc_bootstrap_salt:
  cmd.run:
    - name: INSTALL_DEBIAN_INIT_SCRIPT=1 lxc-unshare -s MOUNT -- chroot "{{ container['rootfs'] }}" sh {{ bootstrap_salt }} git develop
    - unless: test -e {{ container['rootfs'] }}/usr/bin/salt
    - require:
      - file: {{ container['rootfs'] }}{{ bootstrap_salt }}
```