Ignore TERMINATED instances

When an instance is removed, its status become `TERMINATED`.
As stated in [GCE Documentation](https://developers.google.com/compute/docs/instances#checkmachinestatus):

> TERMINATED - The instance either failed for some reason or was shutdown. This is a permanent status, and the only way to repair the instance is to delete and recreate it.

So we need to ignore instances with such a status.

Closes #3.

src/main/java/org/elasticsearch/cloud/gce/GceComputeService.java

@@ -58,6 +58,10 @@ static final class Fields {
         private static final String VERSION = "Elasticsearch/GceCloud/1.0";
     }
 
+    static final class Status {
+        private static final String TERMINATED = "TERMINATED";
+    }
+
     private List<DiscoveryNode> discoNodes;
     private TransportService transportService;
     private NetworkService networkService;
@@ -114,6 +118,13 @@ private List<DiscoveryNode> buildNodes(String project, String zone) throws IOExc
 
             String status = instance.getStatus();
 
+            // We don't want to connect to TERMINATED status instances
+            // See https://github.com/elasticsearch/elasticsearch-cloud-gce/issues/3
+            if (Status.TERMINATED.equals(status)) {
+                logger.debug("node {} is TERMINATED. Ignoring", name);
+                continue;
+            }
+
             // see if we need to filter by tag
             boolean filterByTag = false;
             if (tags.length > 0) {