Update git submodules

* Update docs/submodules/armband from branch 'stable/fraser'
  - [salt] Retire Armband Saltstack repos

    Provided we take care of the minor consequences of relying strictly
    on upstream Saltstack amd64 repos on arm64 (since we don't use RAET,
    this means merely installing `python-futures` as a dependency of
    `python-tornado` on all minions), we can finally drop our Salstack
    arm64 repository.

    This implies installing python-futures on:
    - FN and VCP VMs (handled via lib.sh pre-patching);
    - baremetal nodes (handled via MaaS Curtin template for arm64 now);

    While at it, also handle other minor patch cleanup operations:
     * drop `system.linux.system.repo.mcp.armband`, already covered by:
       - lib.sh pre-patching for FN and VCP VM images;
       - maas.region.repositories for baremetal nodes;
     * switch glusterfs PPA repo arch from 'amd64,arm64' to None;
     * drop obsolete patch handling `mcelog` on AArch64;

    [1] https://github.com/salt-formulas/salt-formula-maas/pull/26

    JIRA: ARMBAND-382

    Change-Id: I34f3bd70ebf10395e8d4e70498ff3a144b4d9c3b
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
    (cherry picked from commit d680c5fb86c3fd4b25e7b7f766064a6ef4bf9a3b)

diff --git a/docs/submodules/armband b/docs/submodules/armband
index c70aa35..cf421ba 160000 (submodule)
--- a/docs/submodules/armband
+++ b/docs/submodules/armband
@@ -1 +1 @@
-Subproject commit c70aa3538ef9b1da9b360ae3ba7112d8289ab876
+Subproject commit cf421ba6f593ad1d7c926db618657f8bf66d01a7