[autotest] Cleanup the logic in determine if SSP should be used.

We have 3 parameters related to ssp:
require_ssp: True if test requires SSP
warn_no_ssp: True if scheduler failed to locate a drone supporting SSP, and
             wants to log a warning in autoserv log.
ssp_url: Url to SSP on devserver, either resolved from host attribute
         job_repo_url or staged within autoserv process.

This change tries to merge the checks into a single use_ssp variable to stop
the logic being used in multiple places, thus reduce the chance of
introducing bugs.

BUG=chromium:453624
TEST=local run test

Change-Id: Ib731d073cc27331f72fd8bf72276c6d58dbe7804
Reviewed-on: https://chromium-review.googlesource.com/264393
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
1 file changed