Configuration-as-code-plugin: Cleanup obsolete "slave" and "master" terminology from JCasC demos and docs

[oleg-nenashev]

The Jenkins project now uses "agent"and "controller" as official terms. See https://issues.jenkins-ci.org/browse/JENKINS-42816 and https://groups.google.com/forum/#!topic/jenkinsci-dev/CLR55wMZwZ8 for the reference. There are some occurrences which could be cleaned up:

• Docker image names
• Configuration entries for plugins which now support "agent"/"controller" aliases.
• Documentation text

There is no need to submit all patches in a single pull request. Contributors can cleanup terminology incrementally and submit smaller PRs.

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Description

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[spielers]

@oleg-nenashev I would like to work on this issue. I have pretty much-working knowledge of documentation I will be happy to work on

[eshack94]

I'm also interested in helping with this. In an effort to keep the PRs small and manageable, is it okay to submit documentation PRs first?

[oleg-nenashev]

Sorry for missing comments @spielers @eshack94 , my GitHub notification inbox was flooded :( Next time please do not hesitate to just take the issue and start working on that, I check my review dashboard almost every day