Make it easier to use YAML configuration to turn off a logger

A level named off is used to disable logging for a particular logger.
YAML interprets off as false, leading to a failed attempt to get the
LogLevel for FALSE. A workaround is to quote the level, i.e. use "off"
rather than off.

This commit updates LoggingApplicationListener to coerce the string
false back to the level off.

Closes gh-3631
See gh-3628

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

@@ -246,7 +246,12 @@ public class LoggingApplicationListener implements SmartApplicationListener {
 				name = null;
 			}
 			level = environment.resolvePlaceholders(level);
-			system.setLogLevel(name, LogLevel.valueOf(level.toUpperCase()));
+			if (Boolean.toString(false).equalsIgnoreCase(level)) {
+				system.setLogLevel(name, LogLevel.OFF);
+			}
+			else {
+				system.setLogLevel(name, LogLevel.valueOf(level.toUpperCase()));
+			}
 		}
 		catch (RuntimeException ex) {
 			this.logger.error("Cannot set level: " + level + " for '" + name + "'");