Add a CLI option to control logging level (#1000)

* Add CLI option to control logging level

* Update the property name

modules/openapi-generator-cli/src/main/resources/logback.xml

@@ -24,7 +24,7 @@
         <appender-ref ref="STDOUT"/>
         <appender-ref ref="STDERR"/>
     </logger>
-    <logger name="org.openapitools" level="info">
+    <logger name="org.openapitools" level="${log.level:-info}">
         <appender-ref ref="STDOUT"/>
         <appender-ref ref="STDERR"/>
     </logger>