adding provisioner id to task apis. Taking or finishing a task
with a provisioner that does not exist will not be allowed.
Also refactoring to clean up code around the request by making
the requests actual objects instead of working directly with json.
make sure consumer id is unique.
update changelog.
Merge remote-tracking branch 'origin/develop' into feature/task-endpoints-add-provisionerid

Conflicts:
	server/src/test/java/com/continuuity/loom/http/LoomServiceTestBase.java
	server/src/test/java/com/continuuity/loom/scheduler/SchedulerTest.java
refactoring common request body decode logic and error handling
into a common helper class.
Merge pull request #284 from continuuity/feature/task-endpoints-add-provisionerid

adding provisioner id to task apis. Taking or finishing a task