Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Fix default IPv4 mask for IxNetwork traffic config

    IP mask is set depending on the IP range provided by the user in the
    traffic profile. In case of single IP in range, IPV4LENGTH value is
    used as mask should cover all address bits.

    JIRA: YARDSTICK-1478

    Change-Id: I5ad40d1f3def52ebcda4400b708b71faf11e12ab
    Signed-off-by: Serhiy Pshyk <serhiyx.pshyk@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index a4084d2..56b4d5d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit a4084d27de22c64ecb690369c7bed49175703adf
+Subproject commit 56b4d5de28bd8f491cea077e21811830959b6ea8