autotest: Move iperf_config.py to server/cros/network

TEST=git grep 'wlan import iperf_config' has no output
BUG=chromium:258537

Change-Id: Iccc6bf60cb14e15bc4a105ebd07d79468a0e1c78
Reviewed-on: https://gerrit.chromium.org/gerrit/61358
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
12 files changed