add constant only used once to make it harder to read the code

src/main/java/org/elasticsearch/bootstrap/Bootstrap.java

@@ -92,8 +92,14 @@ public class Bootstrap {
         }
     }
     
+    /** 
+     * option for elasticsearch.yml etc to turn off our security manager completely,
+     * for example if you want to have your own configuration or just disable.
+     */
+    static final String SECURITY_SETTING = "security.manager.enabled";
+
     private void setupSecurity(Settings settings, Environment environment) throws Exception {
-        if (settings.getAsBoolean("security.manager.enabled", true)) {
+        if (settings.getAsBoolean(SECURITY_SETTING, true)) {
             Security.configure(environment);
         }
     }