Jenkins reprepo broken cause of double redirect

The Jenkins packages are provided upstream via the Debian repository
http://pkg.jenkins-ci.org/debian-stable/ . It is then redirected to a
mirrors.jenkins-ci.org which again redirect to a mirror.

reprepo does not support double redirects:

aptmethod redirects already redirected
'http://mirrors.jenkins-ci.org/debian-stable/jenkins_1.480.3_all.deb'
again to
'http://ftp-nyc.osuosl.org/pub/jenkins/debian-stable/jenkins_1.480.3_all.deb'
Multiple redirects currently not supported by repreproThere have been
errors!

This patch replace pkg.jenkins-ci.org to mirrors.jenkins-ci.org to avoid
the double redirection.

Change-Id: I552211e197d65a41ffff6b0ee436167d23c4fb25