Update git submodules

Update git submodules

* Update docs/submodules/functest from branch 'master'
  - Remove the former Dockerfile

    opnfv/functest had been split into multiple containers based on
    Alpine (See Docker Slicing [1]) during Euphrates.

    As ARM is being switching to them, we can safely remove the related
    files.

    [1] https://git.opnfv.org/functest/tree/docs/com/pres/dockerslicing/dockerslicing.md?h=stable/euphrates

    Change-Id: I30294a94cc742ba80a7bc4f35db9330240385bb0
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>