Bug 1383880: parse try config during the decision task; r=ahal

This sets the try_mode property, and parses the try message (if given), early
in the decision task and puts the results into the parameters.

The proximate need is to set optimze_target_tasks for some try modes and not
others. This also replaces the existing logic for parsing messages for certain
kinds, and makes the distinction between the different try modes a little
clearer.

MozReview-Commit-ID: AXJEGLh6pEV

taskcluster/taskgraph/parameters.py

@@ -21,20 +21,16 @@ PARAMETER_NAMES = set([
     'include_nightly',
     'level',
     'message',
-    'morph_templates',
     'moz_build_date',
     'optimize_target_tasks',
     'owner',
     'project',
     'pushdate',
     'pushlog_id',
-    'target_task_labels',
     'target_tasks_method',
-])
-
-TRY_ONLY_PARAMETERS = set([
-    'morph_templates',
-    'target_task_labels',
+    'try_mode',
+    'try_options',
+    'try_task_config',
 ])
 
 
@@ -44,7 +40,7 @@ class Parameters(ReadOnlyDict):
         names = set(self)
         msg = []
 
-        missing = PARAMETER_NAMES - TRY_ONLY_PARAMETERS - names
+        missing = PARAMETER_NAMES - names
         if missing:
             msg.append("missing parameters: " + ", ".join(missing))