Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - [maas] Set boot source selections

    Select xenial image explicitly to avoid getting latest bionic one.

    Change-Id: Iee872b8dd2ce309231829a0ad8e5a3cf1a75f796
    Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 2d2fbcc..4b28a7b 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 2d2fbccbc5ae167319a9af636d96efaab8f9ae32
+Subproject commit 4b28a7bb0097434381e5a0508ee865a2c2fba9c2