Switch to triggering common user configs via a path unit. This is
particularly useful for config drive so that a config drive can be
mounted by something other than the udev triggered services, a bind
mount when running in a container for example.
A workaround for https://github.com/coreos/coreos-cloudinit/issues/86
Longer term cloudinit needs to be fixed to not corrupt the system when
multiple config sources are being used. We've pretty much gotten this
far without this coming up because most configs don't conflict so badly.
The current cloudinit implementation blocks when starting units which
causes it to deadlock the boot process if a system cloud config starts a
user cloud config because the user configs want to run after system is
done. Until cloudinit switches to non-blocking calls user configs will
go back to just depending on coreos-setup-environment.service.
