nixos: add services.jenkins.jobBuilder option

This option allows to define (declarative) Jenkins jobs, using Jenkins
Job Builder (JJB) as backend.

Example:

  services.jenkins = {
    enable = true;
    jobBuilder = {
      enable = true;
      yamlJobs = ''
        - job:
            name: jenkins-job-test
            builders:
              - shell: echo 'Hello world!'
      '';
    };
  };

Jobs can be defined using YAML, JSON and Nix.

Note that it really is declarative configuration; if you remove a
previously defined job, the module will remove the jobdir under
$JENKINS_HOME.

Jobs managed through the Jenkins WebUI (or by other means) are not
touched by this module.

Changes v1 -> v2:
* add nixJobs
* let jsonJobs take a list of strings (allows merge)
* 4 space indent in shell code