Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  to 502e0f5fbe1925c79ecc52175f1aac342b53d7fb
  - Rewrite Functest verification job

    It deeply simplifies the overall logic and prepare a possible next
    integration in the gate multijobs.

    It's also needed by "Check spelling in rst files" [1] which asks for
    the 'enchant' C library [2].

    [1] https://gerrit.opnfv.org/gerrit/c/functest/+/68643
    [2] https://build.opnfv.org/ci/job/functest-verify-unit-tests-and-docs-master/2459/console

    Change-Id: I07cd1f6b008e5b121d5d9852112f5db756c87da4
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>