Allow to log easily the job's nodelist in performance log

[vkarak]

Currently, this can be done using a custom test field:

class MyTest(rfm.RegressionTest):
    ...
    @rfm.run_after('run')
    def set_nodelist(self):
        self.mynodelist = self.job.nodelist

And then use %(check_mynodelist)s as a format string.

It would be much easier if we supported directly %(check_nodelist)s or %(check_job_nodelist)s or even better support any job attribute to be logged with %(check_job_ATTR)s.