[JUJU-1969] Fix channel during imports

[juanmanuel-tirado]

During the import operation the channel attribute for charms was ignored. It looks like a hardcoded latest/stable value remained there. In this change, we obtain the channel from the response as done with the other fields. This should fix #95.

Following the example mentioned at #95, you can use these steps for testing.

Compile the terraform provider and make it available as usual.

 $ go build
 $ cp terraform-provider-juju <path-to-terraform>/.terraform.d/plugins/registry.terraform.io/juju/juju/0.4.1/linux_amd64/terraform-provider-juju_v0.4.1
 $ cd <path-to-wherever-you-put-the demo.tf>

Using microk8s create a model and a redis deployment.

juju add-model newmodel
juju deploy redis-k8s --channel=edge -m newmodel

Use the following terraform plan:

terraform {
   required_providers {
     juju = {
       source  = "juju/juju"
       version = "0.4.1"
     }
  }
}

provider "juju" {}

resource "juju_model" "development" {
  name = "newmodel"
}

resource "juju_application" "redis_cache" {
  name  = "redis-cache2"
  model = juju_model.development.name

  charm {
    name    = "redis-k8s"
    channel = "edge"
  }

  units = 1
}

Initialize a terraform environment and import the redis application.

terraform init
terraform import juju_application.redis_cache newmodel:redis-k8

Now the plan should not indicate any changes in the channel attribute:

terraform plan
....
# juju_application.redis_cache must be replaced
-/+ resource "juju_application" "redis_cache" {
      - config = {} -> null
      ~ id     = "newmodel:redis-k8s" -> (known after apply)
      ~ name   = "redis-k8s" -> "redis-cache2" # forces replacement
        # (3 unchanged attributes hidden)

      ~ charm {
            name     = "redis-k8s"
          ~ revision = 19 -> (known after apply)
          ~ series   = "focal" -> (known after apply)
            # (1 unchanged attribute hidden)
        }
    }

  # juju_model.development will be created
  + resource "juju_model" "development" {
      + id   = (known after apply)
      + name = "newmodel"
      + type = (known after apply)

      + cloud {
          + name   = (known after apply)
          + region = (known after apply)
        }
    }

[cderici]

LGTM. Typo in QA Steps threw me off a bit :)

terraform init
terraform import juju_application.redis_cache newmodel:redis-k8 <----------------------- k8s