[WIP] Support for canary releases with EXTERNAL deployments

[jfranzoi]

almost done with what we started discussing in #228 (and sorry, it starts from another PR still waiting for approval, #227. still WIP since it lacks some more documentations on README.

added support for detecting deployment controllers other than default ECS (see $DEPLOYMENT_CONTROLLER abstraction). implemented EXTERNAL, both on force new deployment and complete service deployment. CODE_DEPLOY is not supported (and I guess it would never be, since you'd interact with code-deploy itself).

few more arguments, to configure the canary deployment process:

• --task-set-file to optionally use file for task set
• --canary-percent to choose canary deployment size (by default, 25% of primary deployment's task set)
• --canary-confirmation to choose a strategy to confirm (or abort) canary deployment, with few provided ones (interactively ask to user, wait for timeout, or simply proceed) and a custom one, intended as a bash script to be executed (and proceeding on 0 exit code)