Support rest-client 2.0.0
The Gemfile since #13698 limits Foreman to rest-client ~> 1.8.0, but 2.0.0 should be compatible and work fine, with the exception of tests which need changes to construct fake responses. The dependency should be relaxed to permit 2.x.
fixes #15573 - support rest-client 2.x
Add a test helper that understands how to create fake responses for both
supported 1.8 and 2.x versions.