Make jpeg quality configurable.

lib/units/device/plugins/screen/stream.js

@@ -228,7 +228,11 @@ module.exports = syrup.serial()
 
     FrameProducer.prototype._startService = function() {
       log.info('Launching screen service')
-      return minicap.run(util.format('-S -P %s', this.frameConfig.toString()))
+      return minicap.run(util.format(
+          '-S -Q %d -P %s'
+        , options.screenJpegQuality
+        , this.frameConfig.toString()
+        ))
         .timeout(10000)
     }