Cluster member specs in YAML

e.g.
```
services:
 - serviceType: brooklyn.entity.group.DynamicCluster
   id: test-entity-cluster
   name: TestEntityCluster
   initialSize: 2
   memberSpec:
     $brooklyn:entitySpec:
       type: brooklyn.test.entity.TestEntity
       brooklyn.config:
         test.confName: yamlTest
```

Commits a71fdf8 and 70363e1 are in both this and #1212. Don't think it makes a huge difference which is merged first. I'll rebase 1212 if it's a problem.