Update git submodules

* Update docs/submodules/fuel from branch 'stable/euphrates'
  - docs: Add details to the installation instruction

    Added some text details to the diagrams from
    https://gerrit.opnfv.org/gerrit/#/c/46801/

    Change-Id: Ib761ca2a63e7c9e267c3c5c7bbe659d42bddea5e
    Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>
    (cherry picked from commit 6a543cac38e7b405fac489aee4ae3e357f250574)

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index a0a060a..12c1749 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit a0a060a131fb8141d0a1210892443caef250b7b5
+Subproject commit 12c1749e6bd339dc23f155d6990e30f54245c88d