core: Fix QuartzException when Ctrl+C engine

When shuting down engine in dev environment using Ctrl+C
exceptions are printed to the screen because of Quartz shutdown.
This patch validates that the scheduler can indeed reschedule a job before rescheduling it.

Exception Dump is:
failed to reschedule the job: org.quartz.SchedulerException: The Scheduler has been shutdown.
at org.quartz.core.QuartzScheduler.validateState(QuartzScheduler.java:749) [quartz.jar:]
at org.quartz.core.QuartzScheduler.rescheduleJob(QuartzScheduler.java:1060) [quartz.jar:]
at org.quartz.impl.StdScheduler.rescheduleJob(StdScheduler.java:312) [quartz.jar:]
at org.ovirt.engine.core.utils.timer.SchedulerUtilQuartzImpl.rescheduleAJob(SchedulerUtilQuartzImpl.java:331) [scheduler.jar:]
at org.ovirt.engine.core.utils.timer.FixedDelayJobListener.jobWasExecuted(FixedDelayJobListener.java:91) [scheduler.jar:]

Change-Id: I3fcdc3aa51bbcdf817363af6431db488501aa401
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>