core:No order of jobs via start time under..

No order of jobs via start time under /api/jobs

Adding order by start_time DESC in GetAllJobs SP
Adding an index for the start_time field in the jobs table

Change-Id: I3a0af67e65b54939f4b245169ebfc8c9eddb3cef
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988361
Signed-off-by: Eli Mesika <emesika@redhat.com>