fixes #259: disable file logging by setting system property "disable-file-logging"

So no emtpy Logfile and no directory will be created.

Mustang-CLI/src/main/resources/logback.xml

@@ -7,7 +7,8 @@
     </encoder>                                                                                                                                                                                                                                
   </appender>
 
-
+  <if condition='!isDefined("disable-file-logging")'>
+    <then>
       <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
         <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
           <!-- Daily rollover -->
@@ -21,6 +22,8 @@
           <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
         </encoder>
       </appender>
+    </then>
+  </if>
 
   <!-- Configure so that it outputs to both console and log file -->
   <root level="INFO">