Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge changes from topics 'YARDSTICK-875', 'YARDSTICK-874'

    * changes:
      Check if multiverse repository is available in Ubuntu
      Specify supported architectures for Ubuntu backports repository

  - Check if multiverse repository is available in Ubuntu

    In Ubuntu, multiverse repository is needed to install the image
    dependencies [1]. If this repository is not enabled, it should be
    added as part of the role "add_custom_repos".

    [1] https://github.com/opnfv/yardstick/tree/master/ansible/roles/install_image_dependencies

    JIRA: YARDSTICK-875

    Change-Id: Ie37ebf87bf245185447f2b675568af9bd38219c0
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>
  - Specify supported architectures for Ubuntu backports repository

    Currently Trusty (Ubuntu 14.04 LTS) and Xenial (Ubuntu 16.04 LTS) doesn't
    have support for arch=ARM64 in backports repository:
    - http://archive.ubuntu.com/ubuntu/dists/xenial-backports/
    - http://archive.ubuntu.com/ubuntu/dists/trusty-backports/

    During the Yardstick image building process, the repository is added to the
    APT repository list. In order to avoid errors during the repository retrival,
    the available architectures must be specified [1].

    [1]https://github.com/opnfv/yardstick/blob/stable/euphrates/ansible/roles/add_custom_repos/templates/sources.list.j2#L4

    JIRA: YARDSTICK-874

    Change-Id: I7774189fbdd7d74ff5df29526ba0da2693b3b53a
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index d93f0b4..edb1b87 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit d93f0b4e79bcf822dfb264a82e879f45038628d1
+Subproject commit edb1b87fae318501b853ef02a7aa7dffddfe1e48