adding dateutil to the image, as i do not know why RDEPENDS does not ensure that dateutil exists in the rootfs

tried to add python-vcstools via dependency in python-rosinstall but it does not do and then added it the core-image instead; tuned layout and order in core-image

This reverts commit aa0c0ec99370f6ffcd19e9c5f3c9cf1a7cf65a76.

This reverts commit 1fb1afe23d78b45711f8435d64eadf1597847c8c.
rospkg-dev does not solve the issue. It is still unclear which files are missing or which pathes are set wrong.