Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Standalone XML machine type is not longer valid"
  - Standalone XML machine type is not longer valid

    Current XML machine type definition [1] is not longer valid. According to
    [2], the default machine type should be "pc-i440fx-xenial".

    NOTE: StandAlone physical nodes are deployed using Ubuntu OS.

    [1]https://github.com/opnfv/yardstick/blob/master/yardstick/benchmark/contexts/standalone/model.py#L48
    [2]https://wiki.ubuntu.com/QemuKVMMigration#Upgrade_machine_type

    JIRA: YARDSTICK-1192

    Change-Id: I77b1379aaf416b1225e91af2c3ac746cb101f774
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>