Update git submodules

* Update docs/submodules/fuel from branch 'stable/euphrates'
  - salt: Use apt-mk '2017.12' distribution

    'nightly' repo dist from apt-mk is broken, so switch to '2017.12'.

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

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index dac83c6..e3bc5ad 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit dac83c66fa36ce532f68710a630719b6d76a4702
+Subproject commit e3bc5ad8ab4ace79f79ca3492beb76e6af8e0369