Verify mappings status in WireMock test container on startup and fail by default when there're no mappings

[oleg-nenashev]

This is an alternative to #69 for #28 that uses the Wait Strategy to verify the response and say the container is not ready if there are no mappings. Unfortunately it is not going help with #28, bacause there is no way to abort the wait loop from the predicate.

Submitter checklist

• [ ] Recommended: Join WireMock Slack to get any help in #help-contributing or a project-specific channel like #wiremock-java
• [ ] The PR request is well described and justified, including the body and the references
• [ ] The PR title represents the desired changelog entry
• [ ] The repository's code style is followed (see the contributing guide)
• [ ] Test coverage that demonstrates that the change works as expected
• [ ] For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org

[bitxon]

I like the idea to have validation of empty mapping, but i would allow empty mapping by default and failed container only if developer explicitly asked about that (executed withFailureOnEmptyMappings())