Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Make GenericVNF an abstract class

    GenericVNF class is now an abstract class. Only optional methods are
    implemented. Mandatory methods:
      - instantiate
      - wait_for_instantiate
      - terminate
      - scale
      - collect_kpi.

    JIRA: YARDSTICK-866

    Change-Id: Ia8766f9f98816e11894d1e72b0f3bd573d091d99
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index fa3a7b8..67bd299 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit fa3a7b8d6cadbb2d5e9311cb2da040003f9b9140
+Subproject commit 67bd29933789e7f31c52290fdd43835e54f89eaf