Add mechanism to switch to CI_JOB_TOKEN for authentication

As all modules and hostgroup are in the same group, with the same base cloning rights, we can use CI_JOB_TOKEN for authenticating against gitlab when pulling the fixtures, removing the need to add a shared deploy key, 'AI_CI_SSH_KEY'.

This commit adds this mechanism behind a boolean, use_gitlab_ci_token, but we might want to drop it after a global migration? (to avoid having this shared deploy key).