Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Use "protocol" parameters in iperf3 yaml and task file"
  - Use "protocol" parameters in iperf3 yaml and task file

    JIRA: YARDSTICK-755

    There is a history problem that iperf use udp to set a net protocol.
    This code will change it to protocol.
    so you could use 'tcp','udp' and other protocol.

    Change-Id: I1a101013dfe58165a3ed08aa77f0aa2f73d57a12
    Signed-off-by: Ace Lee <liyin11@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index eab7e0f..5721849 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit eab7e0f4b1ffb895a93107ab28b26ceb3b084e99
+Subproject commit 5721849f3f08c2c1c89354341115c834e510b3d6