Fixed `403 Client Error: Forbidden` during cleanup

[bartoszj]

Hi, once again my pull request regarding 403. So I've fixed exception handling, HTTPError has response property with status_code. In my case I'm receiving status code 403 while fetching job configuration at line 179 managed_jobs = (job for job in jenkins.jobs if tag in job.config). I've added additional method which returns "managed" jobs and catches exceptions if needed. Last thing is performance optimizations:

• Each time job.config is executed a new request is sent to the Jenkins server, now job name is checked before sending any request.
• job.config was executed twice, at line 179 and 186, so two requests were sent. Now it is executed only once and returned in a tuple.
• I've removed job.exists, which was sending request to the Jenkins. If we can get job configuration there is no need to check if the job still exists.

I've executed unit tests using tox so nothing should be broken.

[gvalkov]

Hello again and apologies for my poor response times. On first reading your patch looks quite good. I'll look into merging it asap. Thanks.