Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Test Case: OPNFV_YARDSTICK_TC023: VM availability during live migration"
  - Test Case: OPNFV_YARDSTICK_TC023: VM availability during live migration

    JIRA: YARDSTICK-174

    This live migration test case is based on share storage, default share
    storage is enabled.
    This test case will do some config work. And do live migration and
    calculate the migration time and downtime.

    Change-Id: I6601601edebdd0ac6434ba632b1eba9e9bd4fda0
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>