Add support for Jenkins Configuration as Code

[yahavi]

• [x] This pull request is created in the jfrog/jenkins-jfrog-plugin repository.

Fix #31 Fix #22 Fix https://github.com/jenkinsci/jfrog-plugin/issues/7

• The constructor argument names must match the class variable names. Otherwise the following issues may appear during configuration export:

  io.jenkins.plugins.jfrog.configuration.JFrogPlatformBuilder$DescriptorImpl#jfrogInstances: io.jenkins.plugins.casc.ConfiguratorException: Can't read attribute 'platformUrl' from io.jenkins.plugins.jfrog.configuration.JFrogPlatformInstance@2daa7334

• Add JCasC instructions - https://github.com/yahavi/jenkins-jfrog-plugin/tree/fix-jcasc#jenkins-configuration-as-code

[yahavi]

Since the url member was renamed to platformUrl, shouldn't we add a converter from the old configuration?

Good point! Thanks! We don't need to add a converter. Following your feedback, I restored the class variable from platformUrl to url and I only fixed the constructor and the Jelly.