ENYO-2125: allow forking the GitHub project

Use relative URL's all-the-way up to the GitHub root.  Benefits:

- Keep the ability to clone using both `ssh` and `https` URLs (for
  external developers or the fortunate ones that are working behind
  a transparent http proxy)
- Allow forking the project, because later `clone` operation does not
  require to have forked every submodule.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>