Update git submodules

* Update docs/submodules/functest from branch 'stable/euphrates'
  - Allow building containers from a gerrit change

    It leverages on git command instead of pip git support [1].
    Otherwise it forces pip master which raises side effects in rally [2]

    It is mandatory for implementing Functional testing gating [3]

    [1] https://pip.pypa.io/en/stable/reference/pip_install/#git
    [2] https://gerrit.opnfv.org/gerrit/#/c/40909/
    [3] https://wiki.opnfv.org/display/functest/Functional+testing+gating

    Conflicts:
        docker/core/Dockerfile

    Change-Id: Ia18fee3e60fc08da9b96730ce85d5b7c866144aa
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
    (cherry picked from commit 84f82d02630c22e8054266c8c0a53ffb805c2071)

diff --git a/docs/submodules/functest b/docs/submodules/functest
index abd7a52..7997885 160000 (submodule)
--- a/docs/submodules/functest
+++ b/docs/submodules/functest
@@ -1 +1 @@
-Subproject commit abd7a52b2d8764b074f68e026c4a66191e32b35e
+Subproject commit 79978853fea1eef900db1fe873d77636882183be