Makefile: git submodules should track remotes.

Makefile: git submodules should track remotes.

Instead of binding git submodules to a specific commit, track
their remote origins at `make submodules-init`.

This introduces the risk of upstream changes breaking our patches
more frequently (used to be whenever we bumped submodules by hand),
and requiring rebasing, but that already happens for Fuel@OPNFV.

This represents a major improvement in tracking upstream and moving
away from forking, at the expense of more frequent rebasing, until
we upstream or move to post-installation step all our patches.

For example, since we rebased our modules 1-2 weeks ago, quite
a few patches landed upstream (e.g. [1]) that we missed in our builds.

[1] https://bugs.launchpad.net/fuel/+bug/1587773

Change-Id: I2d9e0877513558c86f68875f03976002f757e5ee
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>