Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - Merge "[submodule] Bump Pharos for maas:machines sync"
  - [submodule] Bump Pharos for maas:machines sync

    Drop duplicate maas:machines definitions which could cause conflicts
    in rare corner cases.

    Slightly refactor j2 template expansion to make `conf.virtual.nodes`
    available during first stage.

    Change-Id: I04d56e346b12c6eb97da5c0c0ab1e3446e5fc1b8
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 546d392..531ca87 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 546d3927a8ecd822b319a8eac1f98983df63eb3a
+Subproject commit 531ca8731f9b61e79131bcfab420742b4fe3b494