Allow setting of system properties from context.xml in addition to setting from command-line.

Accomplished by centralizing calls to System.getProperty(String) and related into new file SystemProperties.java.
There, we first check for existence of system property; if not, we look for property in context.xml
This is done for "application" properties (like hudson.DNSMultiCast.disabled) but not for java properties (like user.name)

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -802,7 +802,7 @@ public class ClassicPluginStrategy implements PluginStrategy {
         }
     }
 
-    public static boolean useAntClassLoader = Boolean.getBoolean(ClassicPluginStrategy.class.getName()+".useAntClassLoader");
+    public static boolean useAntClassLoader = SystemProperties.getBoolean(ClassicPluginStrategy.class.getName()+".useAntClassLoader");
     private static final Logger LOGGER = Logger.getLogger(ClassicPluginStrategy.class.getName());
-    public static boolean DISABLE_TRANSFORMER = Boolean.getBoolean(ClassicPluginStrategy.class.getName()+".noBytecodeTransformer");
+    public static boolean DISABLE_TRANSFORMER = SystemProperties.getBoolean(ClassicPluginStrategy.class.getName()+".noBytecodeTransformer");
 }