bump(coreos-base/update_engine): Source path reorg.

All of the helper script services and systemd files are included in the
update_engine repository now. The pxe service now uses the more generic
'stub' name. Fix install path for dbus config. Remove useless udev rule.
feat(coreos-base/coreos-cloudinit): Create generic template.

Replace coreos-cloudinit-oem.service by a generic service template that
can be used for any path. Add rule to load cloud-configs installed by
the coreos-install utility.
bump(coreos-base/coreos-init): Update coreos-install script.
Merge pull request #504 from marineam/pxe

PXE and related updates