Update git submodules

Update git submodules

* Update docs/submodules/dovetail from branch 'master'
  - Support to specify docker image for each test case

    1. Currently each type of test cases will use one docker image.
    2. For example,
       Functest test cases use opnfv/functest-restapi:euphrates
       Yardstick uses opnfv/yardstick:opnfv-5.1.0
    3. It needs to support to specify the docker image for each test case.
    4. For example,
       vping test cases use opnfv/functest-smoke:euphrates
       sdnvpn uses opnfv/functest-features:euphrates
    5. The benefit of this is that these docker images are well verified
       by Functest CI and from Functest plan, they may not support
       functest-restapi docker image in its future release.

    JIRA: DOVETAIL-638

    Change-Id: I8a30eea2233aeff809af2e241b6c939194397832
    Signed-off-by: xudan <xudan16@huawei.com>