Automatically detect latest Xnat plugins release tag

[biphasic]

Currently the release tag for the xnat-plugins repo is hardcoded in the vars.yml. In case you create new xnat-plugin releases, consider updating the release tag in the vars.yml.
Commented out is a github_releases module in one of the playbooks. Unfortunately it needs a github token to access repos, which cannot be put in cleartext into a public repo (the current one has been automatically revoked). Consider using Ansible Vault to get around this issue.