NovatecConsulting#30 - added a new parameter "isBackendListenerDisabl…

…ed" (default value = FALSE). Now the plugin reports a ERROR-log if connection to InfluxDB is not present and sets the above parameter to the value TRUE thus avoiding write-operations to InfluxDB which cause errors. NOTE: in order to avoid never ending hanging of JMeter in case backend listeners have problems the user should set JMeter property jmeterengine.force.system.exit=true. This will force JMeter to terminate at the end of the test.