Add manifest file for networking dataplane stack solution

Manifest file includes dataplane-stack, DPDK and VPP repos.
The dataplane stack repo points to the gerrit repo to avoid
having to edit it during the development cycle. It will point
to the gitlab repo once the development is completed for the
release.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Change-Id: I72b5d59c795e21c6aaba158cf44b5e8fbfb40b29