Update git submodules

Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - Merge "ovsdpdk: Custom Statistics Tests"
  - ovsdpdk: Custom Statistics Tests

    Functional testcases for the validation of Custom Statistics support
    by OVS. The Custom Statistics feature allows Custom Statistics to be
    accessed by VSPERF.
    These statistics include for example rx_missed_errors, rx_errors,
    tx_errors, rx_mbuf_allocation_errors, rx_crc_errors.
    These testcases require DPDK v17.11, the latest Open vSwitch(v2.9.90)
    and the IxNet traffic-generator.

    JIRA: VSPERF-572

    Change-Id: I30cbe6067d6bdd0fc2a262b86f306b66a77d085f
    Signed-off-by: Richard Elias <richardx.elias@intel.com>
    Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Al Morton <acmorton@att.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>