levant/deploy: skip health checks for task groups without canaries

[pmcatominey]

if a job contains at least one task group with canary deployments configured
and at least one without, the deployment watcher will fail the deployment
as the check for healthy canaries assumed every task group is configured for
canary deployments.

Tested with levant deploy -canary-auto-promote 30 echo.nomad and the following Job specification (add/remove :latest to the image argument to trigger a deployment):

job "echo" {
  datacenters = ["dc1"]
  type = "service"

  group "echo-canary" {
    count = 2

    update {
      canary       = 2
      max_parallel = 2
    }

    task "echo-canary" {
      driver = "docker"
      config {
        image = "alpine"
        command = "tail"
        args = ["-f", "/dev/null"]
      }
    }
  }

  group "echo-standard" {
    count = 1

    update {
      max_parallel = 1
    }

    task "echo-standard" {
      driver = "docker"
      config {
        image = "alpine"
        command = "tail"
        args = ["-f", "/dev/null"]
      }
    }
  }
}