Make boot-stack element more friendly to other distros.
Make boot-stack element more friendly to other distros.

Check if it's an upstart-based distro before doing upstart-specific tasks.

Change-Id: I5ce8589dc5adfb0ea74bf4b573a964d286414644

agroup/tripleo-puppet-elements,qwefi/tripleo-image-elements,radez/tripleo-image-elements,qwefi/tripleo-image-elements,rdo-management/tripleo-image-elements,qwefi/tripleo-image-elements,rdo-management/tripleo-image-elements,agroup/tripleo-puppet-elements,openstack/tripleo-image-elements,agroup/tripleo-puppet-elements,radez/tripleo-image-elements,qwefi/tripleo-image-elements,agroup/tripleo-puppet-elements,qwefi/tripleo-image-elements,openstack/tripleo-image-elements