[WIP] Add optional canary deploy group and check
Adds optional canary_dsh_targets config variable that behaves the same
as dsh_targets (if it's an abolute path, use it, otherwise look for
Adds a canary_check stage to DeployLocal that is yet to be
implemented, but happens post canary-fetch, -promote, and -check and can be
used for extended canary checks.
Currently, after canary deploy, it waits for input from user; however,
idealy, cannary_check could continue to run while waiting for user input.