Extend yml syntax for only and except to support specifying repository path

This allows to limit execution of jobs to specific repository.

For example:

job:
  only:
    - branches@gitlab-org/gitlab-ce
  except:
    - master@gitlab-org/gitlab-ce

The above will run job for all branches on gitlab-org/gitlab-ce, except master.

@dzaporozhets @JobV @vsizov Please review.