Fix java.net.SocketException exception in CouchbaseContainer.stop() (#859)

Motivation
==========
When stop() was called without a prior call to
createCouchbaseEnvironment(), it would throw a SocketException
because stopCluster() would end up trying to initialize the cluster
at the same time the container is being shut down.

Modifications
=============
Call stopCluster() before stopping the container, instead of in
parallel.

Result
======
stopCluster() might still call createCouchbaseEnvironment(),
but the call succeeds and the environment is immediately cleaned up.

Author: dnault

modules/couchbase/src/main/java/org/testcontainers/couchbase/CouchbaseContainer.java

@@ -180,7 +180,8 @@ public List<Integer> getBoundPortNumbers() {
 
     @Override
     public void stop() {
-        Stream.<Runnable>of(super::stop, proxy::stop, this::stopCluster).parallel().forEach(Runnable::run);
+        stopCluster();
+        Stream.<Runnable>of(super::stop, proxy::stop).parallel().forEach(Runnable::run);
     }
 
     private void stopCluster() {

modules/couchbase/src/test/java/org/testcontainers/couchbase/CouchbaseContainerTest.java

@@ -12,4 +12,11 @@ public void shouldUseCorrectDockerImage() {
         Assert.assertEquals(CouchbaseContainer.DOCKER_IMAGE_NAME + CouchbaseContainer.VERSION,
             couchbaseContainer.getDockerImageName());
     }
+
+    @Test
+    public void shouldStopWithoutThrowingException() {
+        CouchbaseContainer couchbaseContainer = new CouchbaseContainer();
+        couchbaseContainer.start();
+        couchbaseContainer.stop();
+    }
 }