[docs] local docs building for missing submodules

[docs] local docs building for missing submodules

Some projects already moved away from OPNFV Docs git submodules.
The proposed hack has a couple of problems:
- it still downloads all of OPNFV docs + submodules (recursively!);
- it uses Jenkins to handle gerrit clone/fetch, just to nuke it and
  clone the current project again;
But:
- it is a small change, to be replaced when global-jjb will support
  gsutil upload for docs preview;
- it provides a locally built preview document instead of the old
  (complete) doc preview;

Change-Id: I7426679187eb1cfabb181c59da61f3b794e50e5f
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>