Bug 1455448: Don't include firefox in the decision task index; r=dustin,mshal,rail,Callek a=release

The decision task is used for everything built as part of a push (for
mozilla-central, this is firefox, devedition and fennec, as well as tasks that
aren't strictly part of any product). Thus, having `firefox` encoded as part of
the decision task doesn't make sense.

This changes the route from
    index.gecko.v2.${repository.project}.latest.firefox.decision
to
    index.gecko.v2.${repository.project}.latest.taskgraph.decision
while leaving the latter for backwards compatability with tools that expect it.

Differential Revision: https://phabricator.services.mozilla.com/D996

taskcluster/taskgraph/parameters.py

@@ -188,7 +188,9 @@ def load_parameters_file(filename, strict=True):
         if filename.startswith("task-id="):
             task_id = filename.split("=")[1]
         elif filename.startswith("project="):
-            index = "gecko.v2.{}.latest.firefox.decision".format(filename.split("=")[1])
+            index = "gecko.v2.{project}.latest.firefox.decision".format(
+                project=filename.split("=")[1],
+            )
             task_id = find_task_id(index)
 
         if task_id: