Update git submodules

Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - Merge "LTD: Expand requirements of Soak/Long Duration Tests"
  - LTD: Expand requirements of Soak/Long Duration Tests

    In Project discussion on Jan 17, 2018 including review of RFC 8204,
    we identified the following five areas to improve the Soak tests.
    See the JIRA ticket for details.

    JIRA: VSPERF-557

    Change-Id: I5314c283a383c57d99863fcf7262c4f28e1c12a8
    Signed-off-by: Al Morton <acmorton@att.com>
    Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
    Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>
    Reviewed-by: Richard Elias <richardx.elias@intel.com>