Update git submodules

Update git submodules

* Update docs/submodules/functest from branch 'stable/euphrates'
  - Force requests===2.13.0

    It overrides OpenStack's upper-constraints to allow rally to work vs
    a self signed certificate.

    It will be pushed to the right OpenStack project after the patch is
    confirmed by CI. Euphrates will be released too early regarding the
    full upstream cycle required.

    JIRA: FUNCTEST-876

    Change-Id: I52656f8b98fced103f13dde039082806636c680a
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
    (cherry picked from commit fedc26596ada9d35bb49f08259a7e5ba4cf13ae7)