Fix canary images, add canary dashboard
Fix canary images, add canary dashboard

This fixes the canary jobs so they're back at the :latest tab,
injected by #3671, and adds a canary dashboard.

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid