Add logging.file environment parameter support (#3146)

apm-sniffer/config/agent.config

@@ -41,5 +41,8 @@ agent.service_name=${SW_AGENT_NAME:Your_ApplicationName}
 # Backend service addresses.
 collector.backend_service=${SW_AGENT_COLLECTOR_BACKEND_SERVICES:127.0.0.1:11800}
 
+# Logging file_name
+logging.file_name=${SW_LOGGING_FILE_NAME:skywalking-api.log}
+
 # Logging level
 logging.level=${SW_LOGGING_LEVEL:DEBUG}