Minor improvements to release notes job

Minor improvements to release notes job

Tweak the job so that it is more compact and uses only the necessary
declarations.

- Move generate-release-notes job declaration into autorelease projects
  declaration as it uses the same variables as the rest of the jobs.
- Switch to a 4g VM as generating release notes does not require a large
  system.
- Only keep logs for 7 days as opposed to 30 days.
- Choosing strategy should be default as this is not a job that is
  triggered by Gerrit.
- Set timeout to 10 minutes as the job appears to run in less then 5
  minutes.
- Remove unnecessary parameters from Maven call as the mvn command does
  not deploy anything to Nexus.
- Use global-jjb macros.

Change-Id: I71d8b18656cf941e0ce691493011ffb10e289480
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>