Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - salt: Use apt-mk 'stable' distribution

    'nightly' repo dist from apt-mk is broken, so switch to 'stable'.

    Change-Id: Ie12dfc2a499910b8b98a63886ba16044e66435f5
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 1e1ec25..d1b6119 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 1e1ec2526f063b6787e0980c17179ba0ee1a8174
+Subproject commit d1b6119e288a31e015573363ce77790fec8684df