Improve online gen resolution of host for the download link

[cbornet]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Fix #1517

[wing328]

👍 Local test result looks good:

➜  ~ curl -X POST -H "content-type:application/json" -d '{"openAPIUrl":"https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml"}' http://localhost:8080/api/gen/servers/python-flask

{"code":"db789ee6-efda-4a89-91f5-8b364ee4f9ca","link":"http://12.34.56.78/api/gen/download/db789ee6-efda-4a89-91f5-8b364ee4f9ca"}%

[wing328]

The Travis build error below has nothing to do with this PR:

The command "stack upgrade" failed and exited with 1 during .