JENKINS-57182 ensure that sites are not added twice through JCASC plugin

[daper]

See JENKINS-57182.

Description

I made 2 commits:

• The first one (4a2463b) is just for making it work properly with JCASC plugin.
• The second (25edc9c) is aesthetic, the JCASC plugin is taking the Config.java class name to identify it on the yaml configuration, and there was nothing to identify that chunk of config owns to this plugin's settings. So I've renamed the class and its references from Config to JiraStepsConfig.

That is why I splitted up the PR into 2 commits.

Test cases for this fix

There were no tests for this case previously.

Test plan

• Run the script snippet on a Jenkins' Script Console and verify that only two sites are added.
• Run a Jenkins instance with Configuration as Code Plugin installed and the yaml snippet. Verify that only two sites are added.
• (after 2nd) Restart Jenkins and verify that only 2 sites are configured.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description.
• Appropriate unit or acceptance tests or explanation to why this change has no tests.
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description.
• Reviewed the code.
• Verified that the appropriate tests have been written or valid explanation given.
• If applicable, tested by installing this plugin on the Jenkins instance.

[nrayapati]

@daper Thank you very much for the pr.

[daper]

Hi! Can I speed up somehow this merge? Is here anything still pending?

[jetersen]

I wonder why this steps plugin is not depending on Jira plugin and getting the information from the Jira plugin instead of implementing its own configuration?

[nrayapati]

Thank you @daper for the pull request.

@Casz Config on JIRA plugin is different from what we support on this plugin, so I had to use something different.