VerticleGroup is a list of verticle that share the same context.

when you haved a list of verticle that hope to run on the same context to avoid data race, you can used VerticleGroup

#### Use cases
```java
public class CompositeVerticleGroup extends AbstractVerticleGroup {

@Override
public List verticles() {
return Arrays.asList(new VerticleA(), new VerticleB(), new VerticleC());
}

static class VerticleA extends AbstractVerticle {}

static class VerticleB extends AbstractVerticle {}

static class VerticleC extends AbstractVerticle {}

}

vertx.deployVerticle(CompositeVerticleGroup)
```
VerticleA, VerticleB, VerticleC and CompositeVerticleGroup will run on the same context.

#### Contribution

yes, I haved create an pr