In swarmbucket implement canary of swarming_task_template.json which is used in N% of builds and uses the ref=latest version of kitchen (as opposed to ref=swarming).

The goal is to increase our confidence in releasing changes to the swarming task template or kitchen.

Also, allow a build requester to force using the canary, e.g. pass a special key in the "swarming" build parameter, e.g.

parameters_json = '{"swarming": {"canary_template": true}}'