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.

@ahgittin does this change look sensible to you - need it for a customer to include yaml of `ServiceRestarter` and `ServiceReplacer`, similar to what's done in code in https://github.com/brooklyncentral/brooklyn/blob/master/examples/simple-nosql-cluster/src/main/java/brooklyn/demo/WideAreaCassandraCluster.java#L61-63