[WFLY-3268] Increase memory due to clustering testuite OOM failures

Some of the clustering tests
```
org.jboss.as.test.clustering.cluster.ejb.stateful.StatefulTimeoutTestCase(SYNC-tcp).timeout
org.jboss.as.test.clustering.cluster.ejb.stateful.StatefulFailoverTestCase(SYNC-tcp).nestedBeanFailover
org.jboss.as.test.clustering.cluster.singleton.SingletonTestCase(SYNC-tcp).testSingletonService
```
are failing due to OOM
```
ERROR [org.infinispan.topology.ClusterTopologyManagerImpl] (transport-thread-4) ISPN000230: Failed to start rebalance for cache default-host/stateful-failover: java.lang.OutOfMemoryError: unable to create new native thread
```

Let's see if this helps. As Tom says, the systems have 3-4 GB RAM, so this should be alright there.

https://issues.jboss.org/browse/WFLY-3268