Adding autotest to simulate ethernet plug and unplug

Test uses servo to simulate plug and unplug of the ethernet
port.  This replaces functionality of network_EthernetStressPlugUnplug
client test.

TEST=Run it and it should work.
BUG=None

Change-Id: Id2a3d54a2e1ee290d71398af84909eceecd462c2
Reviewed-on: https://gerrit.chromium.org/gerrit/39575
Reviewed-by: Kris Rambish <krisr@chromium.org>
Commit-Ready: Stanley Wong <stanleyw@chromium.org>
Tested-by: Stanley Wong <stanleyw@chromium.org>
2 files changed