[autotest] demote import-time logging messages to DEBUG level

Import-time logging messages get emitted before any of the normal
logging handlers can get set up. When a script in the chroot, like
test_that, imports some of these modules (or modules that import them)
this causes confusing-to-end-user logging messages to appear in the
script output.

BUG=None
TEST=None

Change-Id: I1260b87cafbde7390dae5553cee84f899b58c03c
Reviewed-on: https://gerrit.chromium.org/gerrit/65978
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Prashanth Balasubramanian <beeps@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
3 files changed