terminology update: remove the usage of the master and slave terms

The support core plugin is still using the terminology slave for the files / directory / error message it produces. For example the agent's directory are collected under nodes/slave/, agent's remoting version is shown as "Slave Version", ...

Those should be replaced accordingly with the term agent

Addiitonally 'master' must be renamed to 'controller' as per https://issues.jenkins.io/browse/JENKINS-65398