teuthology/suite/run.py, scripts/suite.py: disallow scheduling too many jobs

teuthology/suite/run.py, scripts/suite.py: disallow scheduling too many jobs

Add check_num_jobs() to prevent users from accidentally scheduling too many jobs, like
in rfriedma-2021-06-26_19:32:15-rados-wip-ronenf-scrubs-config-distro-basic-smithi.
JOBS_TO_SCHEDULE_THRESHOLD, set to 500 (most runs have fewer jobs than this),
will disallow users from scheduling more than 500 jobs. Users can schedule
more than 500 jobs by disabling this check using the --disable-num-jobs-check flag.

Signed-off-by: Neha Ojha <nojha@redhat.com>