Fix listening on port 8080 Warning

[i0n-]

When the application is deployed with hard-coded port configuration, appear the Warning:

'App is listening on port 8080, it should instead listen on the port defined by the PORT environment variable.'

This patch look for the PORT environment variable, otherwise set 8080.

[kurtisvg]

Not sure this change is warranted. I think AppEngine always wants port 8080?

@averikitsch - thoughts?

[i0n-]

@kurtisvg I did this to prevent this warning in the console logs viewer :

{
 insertId:  "5d26aeb2000c973148c895ec"  
 labels: {
  clone_id:  "00c61b117c21f3d5d43e23db86e8c2c0964ad67598aa0959da791ccdd718816044908044adf3"   
 }
 logName:  "projects/calculadora-sat/logs/stderr"  
 receiveTimestamp:  "2019-07-11T03:36:18.833405023Z"  
 resource: {
  labels: {
   module_id:  "default"    
   project_id:  "calculadora-sat"    
   version_id:  "north"    
   zone:  "us17"    
  }
  type:  "gae_app"   
 }
 severity:  "WARNING"  
 textPayload:  "App is listening on port 8080, it should instead listen on the port defined by the PORT environment variable. As a consequence, nginx cannot be started. Performance may be degraded. Please listen on the port defined by the PORT environment variable."  
 timestamp:  "2019-07-11T03:36:18.825137Z"  
}

And I will update the code, following your recommendations.

[averikitsch]

@kurtisvg this must be a new warning. The app is suppose to be listening to port 8080. (This error sounds more like a Cloud Run error). I will check in the the GAE team.

[averikitsch]

App engine will respect the user's PORT.
@i0n- Thanks for the PR, will you update your changes to reflect @kurtisvg suggestions?

[i0n-]

@averikitsch Updated!