Bug 1395717 - Use filename source-test tasks are defined in as part of their label, r=dustin

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

taskcluster/taskgraph/transforms/task.py

@@ -55,6 +55,9 @@ task_description_schema = Schema({
     # attributes for this task
     Optional('attributes'): {basestring: object},
 
+    # relative path (from config.path) to the file task was defined in
+    Optional('job-from'): basestring,
+
     # dependencies of this task, keyed by name; these are passed through
     # verbatim and subject to the interpretation of the Task's get_dependencies
     # method.