Add openapi-validator.sh script for reference from API repos.

[gmcculloug]

Part of fixing #201

Move the openapi-validator.sh script to a common location for all the API repos to access.

[dippy-bot]

Checked commit gmcculloug@cce78be with ruby 2.5.7, rubocop 0.69.0, haml-lint 0.28.0, and yamllint
0 files checked, 0 offenses detected
Everything looks fine. 🍪

[lindgrenj6]

I think we need to add .travis_scripts/ to the gemspec here:

insights-api-common-rails/insights-api-common.gemspec

Line 14 in bc964c7

spec.files = Dir["{app,config,db,lib,spec/support}/**/*", "LICENSE.txt", "Rakefile", "README.md"]

Looking at it now it doesn't look like it's included. Of course this all depends if we decide to download it from github or use the version bundled with the gem.

[gmcculloug]

@lindgrenj6 Good point. I did end up switching to downloading the script from the master branch so we probably can avoid adding it to the gem unless others thing it should be there for some reason. For now, I'll leave the PR as-is and can update if there is more feedback on this topic.

[lindgrenj6]

LGTM, if we do need to include this in the gem later we need to just update the .gemspec to include .travis_scripts/.