Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "vnf_generic: Fix str object has no attribute items"
  - vnf_generic: Fix str object has no attribute items

    When an IP range is specified in src_ip/dst_ip like:
    src_ip:
       - '152.16.100.180-152.16.100.181'
    yardstick would return str object has no attribute items error.
    This change will return the IP range as is if type is str.

    Change-Id: I3b097777f0d85b0600207157bebba18987ea2275
    Signed-off-by: Dino Simeon Madarang <dinox.madarang@intel.com>
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index c832dd0..578a903 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit c832dd05f20936734faadcd1ffa1dd8d01d08819
+Subproject commit 578a9031d8d0c6f8631db13e89856abbdf8d23d9