Remove jenkins_cli
Document jenkins_command HWRP
Add `jenkins_command` test. Make executor load first.

This commit adds a simple Test Kitchen test for `jenkins_command` as well as
prefixes the `libraries/_exector.rb` file with an underscore to ensure it's
loaded before all other libraries. This also helps visually separate which
libraries are HWRPs and which are "helpers".
Improve documentation on jenkins_command
Refactor jenkins_node into jenkins_slave HWRP [WIP]