Update git submodules

Update git submodules

* Update docs/submodules/functest from branch 'stable/euphrates'
  - Merge "Force python2.7 when creating venv for refstack" into stable/euphrates
  - Force python2.7 when creating venv for refstack

    It applies part of "Fix rally db issue" [1] which is not
    cherry-picked because it mainly focused on rally updates.

    [1] https://gerrit.opnfv.org/gerrit/#/c/47555/

    Change-Id: Ic6850e54860fe2b54a2199e1c3ed89d822ae6284
    Signed-off-by: Linda Wang <wangwulin@huawei.com>