Cleanup terminology in the Jenkins Test Harness lib

[oleg-nenashev]

The Jenkins test Harness library still uses a lot of deprecated "slave" and "master" terminology in its APIs. It would be nice to clean it up by...

• Editing documentation, including Markdown files and Javadoc
• Removing old terminology in methods by deprecating them and introducing the replacement ones (example for JenkinsRule#createSlave() - Terminology Cleanup: Introduce createAgent() and createOnlineAgent() in JenkinsRule #337 )
• Creating new test classes that do not include the obsolete names
• Cleaning up local variables and comments

All of that can be done in small atomic pull requests, contributors are welcome!

More info

• Inclusive Naming sub-project in Jenkins