Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Merge "[docs] update the user guide for "yardstick-operation"" into stable/fraser
  - [docs] update the user guide for "yardstick-operation"

    * Add a new chapter: operation which shows how to use Yardstick and
      how to create test cases
    * Include details on how to reuse and keep VNFs between testcases

    JIRA: YARDSTICK-885
    JIRA: YARDSTICK-886
    Change-Id: I630e701136b9d573539fe69ea0a42018a67d040f
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>
    (cherry picked from commit 5b4cee9606f26f4c208fcd1aec544ddd436b8162)