Update git submodules

Update git submodules

* Update docs/submodules/releng/builder from branch 'master'
  to fafc613b71acb996cd3340e42e401ce8382491a6
  - Merge "Add a swap file to build machines"
  - Add a swap file to build machines

    We have heard this a few times now and again on the TSC mailing list
    https://lists.opendaylight.org/pipermail/tsc/2018-February/008824.html

    It might be beneficial to have some kind of swap available on the
    system. There is a pretty good writeup about swap here:
    https://chrisdown.name/2018/01/02/in-defence-of-swap.html

    Change-Id: Ifc9d57045b54e1b4f9943214e418b41fabf49d04
    Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>