Streamline GitLab API URL variables

The Platform and the Janitor now share a single variable in the Helm values file global.gitlabApiUrl for the GitLab URL. The handling of the GitLab API secret has been reworked too. The Secret resource has been renamed from nmaas-gitlab-janitor-token to nmaas-gitlab-api-token. Additionally it has been moved to global.gitlabApiToken. Old deployment can choose to manually specify global.gitlabApiToken.secret.name to the old value of nmaas-gitlab-janitor-token in order not to create a new secret.