Ensure there is a single bootstrap parent context

Only one neds to be created, and once it is in place in the hierarchy
it should be usable by all the child contexts that get added.

Fixes gh-153

spring-cloud-context/src/main/java/org/springframework/cloud/bootstrap/BootstrapApplicationListener.java

@@ -81,6 +81,11 @@ public class BootstrapApplicationListener
 		if (environment.getPropertySources().contains(BOOTSTRAP_PROPERTY_SOURCE_NAME)) {
 			return;
 		}
+		for (ApplicationContextInitializer<?> initializer : event.getSpringApplication().getInitializers()) {
+			if (initializer instanceof ParentContextApplicationContextInitializer) {
+				return;
+			}
+		}
 		ConfigurableApplicationContext context = bootstrapServiceContext(environment,
 				event.getSpringApplication());
 		apply(context, event.getSpringApplication(), environment);