Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  to ff96cfa79c622f9e17f4f734fb8dfdead619307c
  - Merge "Remove refstack_[platform,compute] from airship testing"
  - Remove refstack_[platform,compute] from airship testing

    RefStack asks for Router testing and forbids skipping any test.
    As the feature is missing on purpose there is nothing wrong to
    skip refstack_compute and refstack_platform.

    All this single tests are verified by tempest_full, tempest_slow
    and tempest_scenario anyway which allow skipping tests.

    Change-Id: I425fd3d71eababde0aca20f158a4217ca3ec99a8
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>