Extract file-based property sources during bootstrap

The bootstrap.properties need to stick with the default properties
during the process of initializing the application.properties
otherwise the ordering ends up wrong because of the addLast()
semantics in an Environment merge. To do this is a bit ugly with
the additional constraint that the default properties has to
remain a MapPropertySource (so that other processors can append
to it if needed). So we created a custom extension of
MapPropertySource that also carries all the bootstrap properties
during the phase where the application.properties are being
processed, but unpacks them as soon as possible afterwards
in an ApplicationContextInitializer, preserving the order, but
making the bootstrap.properties available effectively for the
whole of the startup and initializaion phase of the main context.

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

@@ -28,6 +28,7 @@ import org.junit.rules.ExpectedException;
 import org.springframework.boot.builder.SpringApplicationBuilder;
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.context.ApplicationContextInitializer;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -81,6 +82,24 @@ public class BootstrapConfigurationTests {
 				this.context.getEnvironment().getPropertySources().contains("bootstrap"));
 	}
 
+	@Test
+	public void bootstrapPropertiesAvailableInInitializer() {
+		this.context = new SpringApplicationBuilder().web(false)
+				.sources(BareConfiguration.class).initializers(
+						new ApplicationContextInitializer<ConfigurableApplicationContext>() {
+							@Override
+							public void initialize(
+									ConfigurableApplicationContext applicationContext) {
+								// This property is defined in bootstrap.properties
+								assertEquals("child", applicationContext.getEnvironment()
+										.getProperty("info.name"));
+							}
+						})
+				.run();
+		assertTrue(
+				this.context.getEnvironment().getPropertySources().contains("bootstrap"));
+	}
+
 	/**
 	 * Running the test from maven will start from a different directory then starting it
 	 * from intellij
@@ -105,7 +124,6 @@ public class BootstrapConfigurationTests {
 	@Test
 	public void picksUpAdditionalPropertySource() {
 		PropertySourceConfiguration.MAP.put("bootstrap.foo", "bar");
-		System.setProperty("expected.name", "bootstrap");
 		this.context = new SpringApplicationBuilder().web(false)
 				.sources(BareConfiguration.class).run();
 		assertEquals("bar", this.context.getEnvironment().getProperty("bootstrap.foo"));