Further paring down of leaky proeprty sources in Bootstrap phase

The biggest problem addressed here is one where an
EnvironmentPostProcessor (reasonably) adds entries to the
defaultProperties in the bootstrap context, but then that
property source is not merged with the parent, or is merged
too late (because it only happens when the application context
parent is set). The result would be that things that were activated
during bootstrap would be not be activated in the main context, or
would be activated too late (early enough for beans to bind to but
not for other listeners and post processors to get access to the
additional properties).

See https://github.com/spring-cloud/spring-cloud-sleuth/issues/126

spring-cloud-context/src/test/java/org/springframework/cloud/bootstrap/config/BootstrapConfigurationTests.java

@@ -212,9 +212,9 @@ public class BootstrapConfigurationTests {
 				.sources(BareConfiguration.class).run();
 		assertEquals("main",
 				this.context.getEnvironment().getProperty("spring.application.name"));
-		// The parent is called "application" because spring.application.name is not
+		// The parent has no name because spring.application.name is not
 		// defined in the bootstrap properties
-		assertEquals("application", this.context.getParent().getEnvironment()
+		assertEquals(null, this.context.getParent().getEnvironment()
 				.getProperty("spring.application.name"));
 	}