results: mark --subset and --seed optional
results: mark --subset and --seed optional

before d488b9bd, these params were mandatory. after d488b9bd, they are
optional. because

- these parameters passed in only for "first-in-suite" job
- subset is not mandatory even for "first-in-suite", because there is
  chance that user want to run the full combination of the test matrix.

Signed-off-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@redhat.com>

dmick/teuthology,ceph/teuthology,ktdreyer/teuthology,dmick/teuthology,SUSE/teuthology,SUSE/teuthology,SUSE/teuthology,ceph/teuthology,ktdreyer/teuthology,dmick/teuthology