Several feature addition

I have done some modifications to how Singularity works in some cases:
- #1 It's now possible to have distributed scheduled tasks (several instances),
- #2 Switched to Quartz Cron syntax due to limitations of the Cron to Quartz Cron conversion method (does not supported year). This allows to launch one-off scheduled tasks designed to run only once at a given date,
- #3 SingularityRequest has now an optional rackId parameter which allows to set the rack affinity of the request. The request will only run deployment on the given rackId.

#2 introduces incompatibility with previous version as the cron syntax is different. Other features may change Singularity behavior.