Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - Reset kernel version to xenial image default one

    Virtual compute nodes goes to kernel panic during nova instances ops
    under hwe kernel, so use the default one from xenial image.

    Change-Id: Iae100b68208cc1fb9e43e45f385e762cdbd6573a
    Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 55a4730..578aaad 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 55a4730a03b523ced440a82a7e8bd26e156bb714
+Subproject commit 578aaada4233d2ef311112e684d8a22e64be3af1