Support easy submission of immediate job from REST API

This is a nice feature for job submission go through the REST API
because if a job want to schedule now, it has first to know what's the
time the chronos server is, and give the job a schedule time within
epsilon window. This is a problem when submitting a job from web ui,
because the schedule time by default is current time.

With this patch, one job can be a immediate job by any of below ways:
  * without "schedule" field
  * with empty start time in "schedule" field, e.g. "R1//PT24H"

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>