This adds a `--level` option to taskcluster-graph, and passes the level
supplied from mozilla-taskcluster. It then substitutes that into cache names
for just about every cache (tooltool being the exception, as it verifies hashes
and is thus immune to poisoning). The scopes for these new cache names are
already included in the relevant `moz-tree:level:*` roles.
This also strips `-c6` from cache names; I added this when we were
transitioning from the Ubuntu-based build images, to ensure I got clean caches.
It's no longer necessary.
c20869c86b