add support for scripted pipeline builds in mesa ci
- Refactored repo_set.py and jenkins.py to use jenkins user token authentication
- Added a 'pipeline' attribute to build_spec to support extended jobnames in jenkins
- eg. a job nested under a pipeline will be called 'pipeline/job' in jenkins
- moved reliable_url_open to utils.py to be used in other scripts
- this function now supports user token authentication