Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Make context types constants"
  - Make context types constants

    Make context types constants. These are the current context types:
      - Dummy
      - Heat
      - Kubernetes
      - Node
      - StandaloneOvsDpdk
      - StandaloneSriov

    Any reference to a context type should use the referred constant.

    JIRA: YARDSTICK-1292

    Change-Id: I1ff0085161f88304d71e3ddad7975f9020e0fc20
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>