Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - [maas] Make boot-source-selection ops idempotent

    - add `unless` clause;
    - instead of adding a new boot sources, update the existing one,
      assuming standard MaaS installation (always updates id '1');

    Change-Id: I2db92a3b14d0588095c69545cbb6977ca109613e
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 2e4aa7c..d7ab894 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 2e4aa7c2e95a4eaee40a0a10e107a75494d76a5c
+Subproject commit d7ab894b988d8dd6fe49714e0d8d6077e08ebb16