MediaWiki Canaries: use swagger spec

Summary:
In addition to checking logstash for sudden increases in error-rate,
MediaWiki deployments should be able to query a known set of endpoints
for expected responses. Since this is exactly what
service-checker-swagger does for services given a url, host, and spec
path we can shell out to that as part of a deployment.

Bug: T136839

Reviewers: demon, mmodell, Release-Engineering-Team

Reviewed By: mmodell, Release-Engineering-Team

Subscribers: jenkins, Krinkle

Tags: Release-Engineering-Team

Differential Revision: https://phabricator.wikimedia.org/D1031