Not valid URL: localhost:9000/api. Default to http://localhost.

Description

I got the following warning in a test spec:

[main] WARN org.openapitools.codegen.utils.URLPathUtils - Not valid URL: localhost:9000/api. Default to http://localhost.

I think we may be able to better handle this by automatically appending http:// if it's not found at the beginning of the URL

openapi-generator version

Latest master

Steps to reproduce

Craft a spec with localhost:9000/api and run with any generator

Suggest a fix/enhancement

Update URLPathUtils class to better handle this case for the user.