Add integration with Gating plugin

When Cachet Gating plugin [1] is installed on the Jenkins master, OpenStack cloud plugin could use it to determine if configured OpenStack instance is healthy or not.

If the OpenStack instance is deemed not healthy, OpenStack Cloud plugin would stop OpenStack API requests to prevent flooding OpenStack API with requests (which would very likely fail anyway). After the instance gets healthy again, OpenStack Cloud Plugin would resume API calls to OpenStack automatically.

The OpenStack Cloud configuration page could include an input box to specify the Cachet resource name which should be checked.

[1] https://plugins.jenkins.io/cachet-gating