backport: optionally choose what worker to send tasks to

Conflicts:
	indexing-service/src/main/java/io/druid/indexing/overlord/http/OverlordResource.java
	services/src/main/java/io/druid/cli/CliOverlord.java
Merge pull request #904 from metamx/backport-autoscaling

backport: optionally choose what worker to send tasks to