[autotest] Modify test_that to figure out the board of the remote host.

Test_that currently relies on the user for the --board argument. This cl
teaches test_that to ssh into a DUT and find the board, if one isn't
specified.

TEST=Invoked test_that dummy_Pass with and without a board, with and without a
     resutls dir, on DUTs with different boards.
BUG=chromium:265705
CQ-DEPEND=CL:I50811eb8b0e6b145a190211391c2409913bc8a33

Change-Id: I0b12bb456c8f032266c7530a39f87cefc5d9182b
Reviewed-on: https://chromium-review.googlesource.com/169767
Tested-by: Prashanth B <beeps@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Prashanth B <beeps@chromium.org>
3 files changed