zuul-gateway: convert external events to git based trigger

[connection virtual]
driver=pagure
server=localhost:5000
baseurl=http://localhost:5000

- tenant:
    name: local
    source:
      virtual:
        untrusted-projects:
        - gateway

- pipeline:
    name: check
    trigger:
      virtual:
        - event: pg_pull_request
          action:
            - opened
    success:
      virtual:
        status: 'success'
    failure:
      virtual:
        status: 'failure'

FLASK_APP=gateway.py flask run

$ cat <<EOF>zuul.yaml
- project:
    check:
      jobs:
        - rpm-lint
    vars:
      rpm_url: http://koji.example.com/test.rpm
EOF
$ curl -X POST --header "Content-Type: application/yaml" \
    --data-binary @zuul.yaml http://127.0.0.1:5000/jobs/rpm-lint-test
{"status":"pending"}
$ curl http://127.0.0.1:5000/jobs
{"rpm-lint-test": {"comment": "Build succeeded.\n\n- [rpm-lint](https://sftests.com/logs/1/1/None/check/rpm-lint/5125e93/) : SUCCESS in 40s\n", "status": "success"}