Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - pathspec 'master' did not match any file(s) known to git

    JIRA: YARDSTICK-605

    On stable branch, exec_tests.sh will also try to git checkout master
    branch, which cause error "pathspec 'master' did not match any file(s)
    known to git." This is a remianing bug from the previous release.
    see at:
    https://build.opnfv.org/ci/view/yardstick/job/yardstick-apex-lf-pod1-daily-danube/62/console
    This error occur in line 48.
    But the reason is 'set -e'
    If we set 'set -e':
    'git checkout master && git pull' return 1 but it will go on executing;
    but 'git checkout master' will return 1 and terminate immediately.
    If we do not set, it will return 1 but will terminate immediately.
    Actually we not need 'git checkout master' here because it is already in
    stable/danube branch.
    So I remove it.

    Change-Id: I106a1da28cf2deee90ebcb25adaf638a210928ee
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>