Test using rbenv for foreman-installer tests

[ehelms]

There are a couple implications here:

1. Using a node wide cache for rubygems to speed bundle install up
2. Using shell based rbenv to prevent clashes between instances

[ehelms]

An alternative approach would be to include a check for if a particular environment variable is set (e.g. RUBY_VERSION) then use it unless version is explicitly specified. That would allow not having to pass the ruby version as the first argument everytime if we find that annoying.

[ehelms]

This initial change includes:

• Set of shared library to setup and run in rbenv context
• conversion of foreman-installer-pr-test job to show how it will look
• a simple ruby test job that allows more easily testing changes to how Ruby test code works