Add gitlab_api_version to decide which api version to use

[mstinsky]

Hey, as we discussed in #44 and #43 this pull requests adds an gitab_api_version variable so the user can still decide to use the v3 api. I only tested the resources which are in use in my infrastructure so api version v4 with git_deploy_key and git_webhook, so other resources and the v3 compatibility still needs testing. Also if we are going to merge something like this we need to have a warning for users that are still using gitlab with api v3 to set gitlab_api_version to v3 when upgrading the module.

[etnoy]

Yes, abrader#47

Thanks!

(I had no idea that my comment would show up on the pull request page).