RPM install jenkins.repo file

The RPM spec file installs a yum repo file, if it isn't already there. This causes issues when used in an enterprise environment, where you don't want the system to go to outside sources to get updates.

Suggested installation instructions for red hat systems is to install the repo file manually & the key, and then use yum to install, so this feature isn't even used in most cases: the file is only installed if you don't already have it in place. If you install from the repo in the first place, you've already got the file, and if you don't, you probably don't want the repo installed.

You also don't install the key, so it will likely cause failures if someone hasn't already imported the key.

Reference:

https://github.com/jenkinsci/jenkins/blob/master/rpm/SPECS/jenkins.spec#L87
https://github.com/jenkinsci/jenkins/blob/master/rpm/SPECS/jenkins.spec#L145