[autotest] add a --ssh_options option to autoserv

This CL follows the template of
https://chromium-review.googlesource.com/167415

BUG=chromium:285378
TEST=Unit tests pass. Manually verified that specifying
--ssh_options="-F /dev/null -i /dev/null" caused those flags to be
threaded into ssh commands.

Change-Id: Ie0496862d05d42192d55501fc0ed1c3a6274fe79
Reviewed-on: https://chromium-review.googlesource.com/168099
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
6 files changed