Bug 1249078 - Support grouping and running tasks by tag; r=garndt

We can now define a list of "tags" for a task. Specifying "-j <tag>"
in Try syntax will run all tasks having that tag.

MozReview-Commit-ID: Ih9Z0tRZ5VA

testing/taskcluster/taskcluster_graph/commit_parser.py

@@ -332,9 +332,23 @@ def parse_commit(message, jobs):
             })
 
     # Process miscellaneous tasks.
-    for name, task in sorted(jobs.get('tasks', {}).items()):
+
+    def filtertask(name, task):
         # args.jobs == None implies all tasks.
-        if args.jobs is not None and name not in args.jobs:
+        if args.jobs is None:
+            return True
+
+        if name in args.jobs:
+            return True
+
+        for tag in task.get('tags', []):
+            if tag in args.jobs:
+                return True
+
+        return False
+
+    for name, task in sorted(jobs.get('tasks', {}).items()):
+        if not filtertask(name, task):
             continue
 
         # TODO support tasks that are defined as dependent on another one.