Code review comment for charm-k8s-jenkins-agent:manual-agent

> We're missing a commit message for this MP, so it wouldn't let us merge yet.
>
> I've included some comments inline, but also I had to make the following
> changes to get lint and tests to run (`make test`)
> https://paste.ubuntu.com/p/j3tSHmbmcB/

I addressed all the comments but two:

- The term "master" isn't going away as per their blog post[1], only "slave". So I think we should follow the same terms Jenkins devs are using.
- The interface is still called jenkins-slave, so we should keep that until we update the interface.