Update git submodules

* Update docs/submodules/fuel from branch 'stable/danube'
  - Revert "Revert: Blueprint "bp/distributed-serialization""

    This reverts commit 8b28c44a4be6beef8a71e672f543001bcc0a080f.
    The issue has been fixed in upstream.

    Change-Id: Id4ad8fb7b1ec887f8d8d1d5b18a9b33a73bda10c
    Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>
    (cherry picked from commit 4a360c5076f839c8b700638f9b85c3e630894826)

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 0bc3d7a..68898be 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 0bc3d7a2e72d7b6c48ccb3e3c4512c592658a9b3
+Subproject commit 68898be998129ab6793228000f3f4528783359bf