Increase numberOfNodes in SchemaChangesCCTest

When running this test with newer Scylla, the drop keyspace query times out.
This is because the cluster lacks raft quorum, although the returned
`OperationTimedOutException` does not inform about that.
To observe this you need to enable debug logging and then you may find
message like this one before the exception:
```
[cluster2-nio-worker-3] DEBUG com.datastax.driver.core.Connection - Connection[/127.0.1.2:40269-1, inFlight=0, closed=false] Response received on stream 832 but no handler set anymore (either the request has timed out or it was closed due to another error). Received message is ERROR SERVER_ERROR: group [e4fdf001-521a-11f0-b1fb-7d275bcabf53] raft operation [read_barrier] timed out, there is no raft quorum, total voters count 2, alive voters count 1, dead voters [127.0.1.1]
```
Increasing the number of nodes to 3 allows the cluster to maintain the raft
quorum and does not impact the test itself.

Author: Bouncheck

driver-core/src/test/java/com/datastax/driver/core/SchemaChangesCCTest.java

@@ -33,7 +33,7 @@
 import org.mockito.ArgumentCaptor;
 import org.testng.annotations.Test;
 
-@CCMConfig(numberOfNodes = 2, dirtiesContext = true, createCluster = false)
+@CCMConfig(numberOfNodes = 3, dirtiesContext = true, createCluster = false)
 public class SchemaChangesCCTest extends CCMTestsSupport {
 
   private static final int NOTIF_TIMEOUT_MS = 5000;