fix(rpm) correct openSUSE instructions to ensure autorefresh is enabled on new installations

[dduportal]

Fix #543 by:

• Updating openSUSE instructions to use the same method as for Redhat to add the repository:
  • Tell end user to install prerequisite at first (including wget)
  • Retrieve repository by downloading (with wget) our custom jenkins.repo file. Have not written "upgrade from existing" instruction though, as I expect instructions to move to www.jenkins.io like for redhat.
  • Finally install the Jenkins package
• Remove the jenkins.repo file from RPM sources as it is not used anymore since SuSE/RPM merge specfiles and only build a single rpm for RH/SuSE #430
  • Note: had to fix the build.sh script

Testing done

• Local build of the RPM to ensure there are no errors due to changes in RPM source of build script
• Opened fix(rpm) correct openSUSE instructions to ensure autorefresh is enabled on new installations #710 for end to end testing in Jenkins infra staging environment
  • Verified that the RPM can be installed with the updated instructions in an opensuse/leap container

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[dduportal]

Self merging this one as it only updates HTML instructions and the build/publish process has been verified.

Removing the jenkins.repo from source does NOT change anything in the RPM which do not have the file for 2.538 and 2.539: no user facing change.