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/LocalPluginManager.java

@@ -60,7 +60,7 @@ public class LocalPluginManager extends PluginManager {
     @Override
     protected Collection<String> loadBundledPlugins() {
         // this is used in tests, when we want to override the default bundled plugins with .jpl (or .hpl) versions
-        if (System.getProperty("hudson.bundled.plugins") != null) {
+        if (SystemProperties.getProperty("hudson.bundled.plugins") != null) {
             return Collections.emptySet();
         }