Add placeholder support to LoggingApplicationListener

When logging.level.* is set in external configuration they can now
contain placeholders.

Fixes gh-1680

spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java

@@ -224,7 +224,7 @@ public class LoggingApplicationListener implements SmartApplicationListener {
 		for (Entry<String, Object> entry : levels.entrySet()) {
 			String name = entry.getKey();
 			try {
-				LogLevel level = LogLevel.valueOf(entry.getValue().toString());
+				LogLevel level = LogLevel.valueOf(environment.resolvePlaceholders(entry.getValue().toString()));
 				if (name.equalsIgnoreCase("root")) {
 					name = null;
 				}

spring-boot/src/test/java/org/springframework/boot/logging/LoggingApplicationListenerTests.java

@@ -204,6 +204,18 @@ public class LoggingApplicationListenerTests {
 		assertThat(this.outputCapture.toString(), containsString("testattrace"));
 	}
 
+	@Test
+	public void parseLevelsWithPlaceholder() throws Exception {
+		EnvironmentTestUtils.addEnvironment(this.context, "foo=TRACE",
+				"logging.level.org.springframework.boot=${foo}");
+		this.initializer.initialize(this.context.getEnvironment(),
+				this.context.getClassLoader());
+		this.logger.debug("testatdebug");
+		this.logger.trace("testattrace");
+		assertThat(this.outputCapture.toString(), containsString("testatdebug"));
+		assertThat(this.outputCapture.toString(), containsString("testattrace"));
+	}
+
 	@Test
 	public void parseLevelsFails() throws Exception {
 		EnvironmentTestUtils.addEnvironment(this.context,