Bug 1383880: allow only one optimization per task; r=ahal

It is not at *all* clear how multiple optimizations for a single task should
interact. No simple logical operation is right in all cases, and in fact in
most imaginable cases the desired behavior turns out to be independent of all
but one of the optimizations. For example, given both `seta` and
`skip-unless-files-changed` optimizations, if SETA says to skip a test, it is
low value and should be skipped regardless of what files have changed. But if
SETA says to run a test, then it has likely been skipped in previous pushes, so
it should be run regardless of what has changed in this push.

This also adds a bit more output about optimization, that may be useful for
anyone wondering why a particular job didn't run.

MozReview-Commit-ID: 3OsvRnWjai4

taskcluster/taskgraph/transforms/docker_image.py

@@ -61,9 +61,9 @@ def fill_template(config, tasks):
         # up on level 3 at some point if most tasks use this as a common image
         # for a given context hash, a worker within Taskcluster does not need to contain
         # the same image per branch.
-        optimizations = [['index-search', '{}.level-{}.{}.hash.{}'.format(
-            INDEX_PREFIX, level, image_name, context_hash)]
-            for level in reversed(range(int(config.params['level']), 4))]
+        optimization = {'index-search': ['{}.level-{}.{}.hash.{}'.format(
+            INDEX_PREFIX, level, image_name, context_hash)
+            for level in reversed(range(int(config.params['level']), 4))]}
 
         # Adjust the zstandard compression level based on the execution level.
         # We use faster compression for level 1 because we care more about
@@ -80,7 +80,7 @@ def fill_template(config, tasks):
             'attributes': {'image_name': image_name},
             'expires-after': '1 year',
             'routes': routes,
-            'optimizations': optimizations,
+            'optimization': optimization,
             'scopes': ['secrets:get:project/taskcluster/gecko/hgfingerprint'],
             'treeherder': {
                 'symbol': job_symbol,