Consider moving away from deprecated, problematic terminology

[samdbmg]

The Jenkins project moved away from using the term "slave" in 2016, preferring the term "agent" instead.

This repository talks about Jenkins "slaves" in a few places that wouldn't be that difficult to change:

• Names of tasks, handlers etc.
• The README
• Variables used within the role such as the systemd unit template
• Names of some of the role files

More difficult is the default variables, but defaults/main.yml could be amended to something along these lines to avoid a breaking change:

jenkins_agent_user: {{ jenkins_slave_user }}
jenkins_slave_user: jenkins

Would you be happy to take a PR changing the terminology? There's also the name of the role itself which might be a more difficult change, because I'm not sure how Ansible Galaxy would behave if you renamed it.