results,schedule,woker: persist --seed and --subset in results.log

results,schedule,woker: persist --seed and --subset in results.log

to create a repeatable test suite, in addition to `--seed <SEED>`, we also
need to pass the same `--subset <SUBSET>` to teuthology-suite when
rerunning the failed tests. but it would be handy if teuthology-suite
could remember these settings and recall them when `--rerun <RUN>`.

in this change, we repurpose the last job sending the email to report
the test result to note down the subset and seed used for scheduling the
test suite. these variables are stored in results.log at this moment.

Signed-off-by: Kefu Chai <kchai@redhat.com>