Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Add yardstick.benchmark.contexts.heat.HeatContext._delete_key_file()

    * Move logic for removing key file into its own method
    * Update the log message to be more useful

    JIRA: YARDSTICK-1026
    Change-Id: I8c131720ed91c939698c41ad63d586396fcce1fe
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index a74ad5a..24a8c6d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit a74ad5a1ec1a73389c5983440b2031b0bc72cea1
+Subproject commit 24a8c6d89f150666a37a3f8daba0fce7c17a436a