Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "Create Xtesting stable/fraser"
  - Create Xtesting stable/fraser

    Xtesting is not directly part of Fraser but this stable branch could
    be required for bugfixes.

    stable/pike could have been selected instead but it would have raised
    side effects when syncing requirements or building containers.

    Change-Id: I407f651ba09e27a7c87cc76f7b8ccbd6c0784a48
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>