moving a lot of validation for cluster create requests from
solve time to request time, as well as adding additional checks
for required provider fields.
Merge remote-tracking branch 'origin/develop' into feature/check-cluster-args-early

Conflicts:
	server/src/main/java/com/continuuity/loom/scheduler/task/ClusterService.java
moving validation code to their respective classes instead of
keeping them all in the cluster service.
Merge pull request #432 from continuuity/feature/check-cluster-args-early

moving a lot of validation for cluster create requests from