This allows defining a "job-defaults" key in a yaml file specified by
"jobs-from". Defaults defined here will first be merged into kind specified
defaults (if any), and then tasks defined in the same file will be merged into
the combined defaults.
MozReview-Commit-ID: Jy5m4Pc9umx
This creates a new "job-from" field that contains the relative filename the job was defined
in. The filename is relative to 'config.path'. If the task came from the 'jobs' key defined
in kind.yml, this field will be set to 'kind.yml'.
MozReview-Commit-ID: 9e1tEb6XuZT