Update git submodules

* Update docs/submodules/fuel from branch 'stable/fraser'
  - Merge "[maas] Set boot source selections" into stable/fraser
  - [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>
    (cherry picked from commit 4b28a7bb0097434381e5a0508ee865a2c2fba9c2)

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 5ccfb79..39c9a0b 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 5ccfb79098c726477e9a07dad2a57e0693bf6ba6
+Subproject commit 39c9a0bee27faf0313c79a58103704e94ac91ede