Add support for PubSub emulator host variable

Author: mziccard

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/PubSubOptions.java

@@ -32,6 +32,7 @@ public class PubSubOptions extends GrpcServiceOptions<PubSub, PubSubRpc, PubSubO
   private static final long serialVersionUID = 5640180400046623305L;
   private static final String PUBSUB_SCOPE = "https://www.googleapis.com/auth/pubsub";
   private static final Set<String> SCOPES = ImmutableSet.of(PUBSUB_SCOPE);
+  private static final String EMULATOR_HOST_ENV_VAR = "PUBSUB_EMULATOR_HOST";
   private static final String DEFAULT_HOST = PublisherSettings.getDefaultServiceAddress()
       + ':' + PublisherSettings.getDefaultServicePort();
 
@@ -66,7 +67,8 @@ public PubSubRpc create(PubSubOptions options) {
 
   @Override
   protected String defaultHost() {
-    return DEFAULT_HOST;
+    String host = System.getProperty(EMULATOR_HOST_ENV_VAR, System.getenv(EMULATOR_HOST_ENV_VAR));
+    return host != null ? host : DEFAULT_HOST;
   }
 
   public static class Builder extends