yaml: accept $brooklyn:sensor ref'ing non-entity

- Previously the class referenced in $brooklyn:sensor had to be an 
  entity.
- However, other classes (e.g. policies such as ServiceRestarter) 
  also declare sensors, so relax restriction.
Merge pull request #1339 from aledsage/fix/yamlSensorAcceptAnyClass

yaml: accept $brooklyn:sensor ref'ing non-entity