Call LoggerContext.reset() during logback init

Update `LogbackLoggingSystem` to call the `reset()` method on the
`LoggerContext` before initialization.

This will hopefully reset the context to prevent the same appenders
from being accidentally added more than once.

Fixes gh-1091

spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -96,6 +96,7 @@ public class LogbackLoggingSystem extends AbstractLoggingSystem {
 						+ "or the competing implementation (" + factory.getClass() + ")");
 		LoggerContext context = (LoggerContext) factory;
 		context.stop();
+		context.reset();
 		try {
 			URL url = ResourceUtils.getURL(resolvedLocation);
 			new ContextInitializer(context).configureByResource(url);