Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Heat: add availability_zone"
  - Heat: add availability_zone

    Users would like to ensure placement of VMs on specific compute nodes so
    that the measurements are meaningful. Examples: Measure network
    performance in different scenarios (VMs in same host, in different
    hosts, across fabric, across tenants)

    Example:

    context:
      name: yardstick
      placement_groups:
        pgrp1:
          policy: "availability"
      servers:
        tg_0:
          floating_ip: true
          placement: "pgrp1"
          availability_zone: "zone2"
        vnf_0:
          floating_ip: true
          placement: "pgrp1"
          availability_zone: "zone1"

    Change-Id: I28a757c25ae3f5b3571ab3edd82d51ceba32c302
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index bf049c6..9894640 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit bf049c61b6c11ff0756220b44b73ebbdc101366a
+Subproject commit 9894640fb446b587c621804572eb580886bae7a1