[Java][Client] Generate servers for okhttp-gson

[outscale-mdr]

This PR is to support servers variables in okhttp-json, this is an addon of #4998 because it was missing the okhttp-gson.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

[outscale-mdr]

Can you restart CircleCi pipeline, it fails because it did not have oAuth authorization and the "rerun" button is disable for my user ?

[outscale-mdr]

Thank you @borsch , can you restart circle-ci pipeline ?

[borsch]

Thank you @borsch , can you restart circle-ci pipeline ?

Nope, I don't have such permissions. You can either
a) merge master & push changes again
b) or just close and re-open same pull request again. this will re-trigger pipelines

[outscale-mdr]

@borsch, Can we merge ? Node1 seems to fail for a lot of PR

[borsch]

@borsch, Can we merge ? Node1 seems to fail for a lot of PR

I don't have write permissions. ask somebody from core team https://github.com/openapitools/openapi-generator/#61---openapi-generator-core-team

[outscale-mdr]

cc @wing328 @jimschubert @cbornet @jmini @etherealjoy @spacether

[wing328]

LGTM. Thanks for the PR.