submodule: yardstick.

submodule: yardstick.

Fuel@OPNFV just added the Yardstick plugin.

This plugin is a little different, instead of breaking fuel plugin
code apart from the contents (i.e. fuel-plugin-yardstick separate
from yardstick itself), it holds both fuel plugin metadata and
actual yardstick code in the same repo.

So far, yardstick plugin code itself is arch-independant and should
not be touched.

TODO:
- add yardstick patches for arm64 until we can validate Yardstick;
- submit patches added locally in Armband to upstream
  (Yardstick@OPNFV);

Change-Id: I24bc8858b92c9bdd529a64de833d7b98c06a6ade
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>