Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Convert SLA asserts to raises"
  - Convert SLA asserts to raises

    This commit converts Python assertions to a custom exception in all
    places where SLA validation is checked with an assertion.

    This commit also fixes all emerged pylint errors.

    JIRA: YARDSTICK-966

    Change-Id: If771ed03b2cbc0a43a57fcfb9293f18740b3ff80
    Signed-off-by: Miikka Koistinen <miikka.koistinen@nokia.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 64add9a..6ff516b 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 64add9a435e9455b3bff6971d2717857cdce8193
+Subproject commit 6ff516b16246e18c0c452905d6e0806db4e53cc0