[grid] Grid is ready when Nodes are available

The Grid status endpoint was returning false when
all slots in all Nodes were used. This was accurate
and it could tell users to wait until more new
session requests are sent.

However, we have the session queue now, and even
if all slots in all Nodes are used, the Grid can
still receive new session requests and queue them.

After this change, the Grid status will return true
when the Grid has one or more available Nodes.

Fixes SeleniumHQ/docker-selenium#1167

java/server/src/org/openqa/selenium/grid/router/GridStatusHandler.java

@@ -42,6 +42,7 @@
 
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static java.util.stream.Collectors.toList;
+import static org.openqa.selenium.grid.data.Availability.UP;
 import static org.openqa.selenium.remote.http.Contents.asJson;
 import static org.openqa.selenium.remote.tracing.HttpTracing.newSpanAsChildOf;
 import static org.openqa.selenium.remote.tracing.Tags.EXCEPTION;
@@ -120,7 +121,9 @@ public HttpResponse execute(HttpRequest req) {
         return response;
       }
 
-      boolean ready = status.hasCapacity();
+      boolean ready = status.getNodes()
+        .stream()
+        .anyMatch(nodeStatus -> UP.equals(nodeStatus.getAvailability()));
 
       List<Map<String, Object>> nodeResults = status.getNodes().stream()
         .map(node -> new ImmutableMap.Builder<String, Object>()