[JUJU-1938] Ignore similar config values when reading config.

[juanmanuel-tirado]

When reading an existing application now:

• Ignore configuration elements with default values. They are not relevant.
• Ignore configuration values similar to the already existing entries. If the read entry differs, this value will overwrite the existing value in the plan.

This Fixes #96

Follow a similar scenario to the one described in #96.

1. Prepare a plan

provider "juju" {}

resource "juju_model" "test" {
  name = "test"
}

resource "juju_application" "indico" {
  name  = "indico"
  model = juju_model.test.name

  charm {
    name = "indico"
  }

  config = {
    indico_no_reply_email = "test@example.com"
  }
}

2. Create the corresponding environment in Juju 2.9.37

juju bootstrap localhost controllertest
juju add-model test
juju deploy indico --config indico_no_reply_email="test@example.com"

3. Run terraform

terraform init --plugin-dir=/whereveryourpluginsare/ -upgrade
terraform import juju_application.indico test:indico

4. Check that the formed plan does not show any changes in the configuration

Terraform will perform the following actions:

  # juju_application.indico will be updated in-place
  ~ resource "juju_application" "indico" {
        id     = "test:indico"
        name   = "indico"
        # (4 unchanged attributes hidden)

      ~ charm {
          ~ channel  = "stable" -> "latest/stable"
            name     = "indico"
            # (2 unchanged attributes hidden)
        }
    }

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

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

Plan: 1 to add, 1 to change, 0 to destroy.

Notice that only the channel will be modified to be latest/stable.

5. (Additional) Update the configuration using the juju cli and refresh

juju config indico indico_no_reply_email="otheremail@example.com"
terraform plan
....
# juju_application.indico will be updated in-place
  ~ resource "juju_application" "indico" {
      ~ config = {
          ~ "indico_no_reply_email" = "otheremail@example.com" -> "test@example.com"
        }
        id     = "test:indico"
        name   = "indico"
        # (3 unchanged attributes hidden)

      ~ charm {
          ~ channel  = "stable" -> "latest/stable"
            name     = "indico"
            # (2 unchanged attributes hidden)
        }
    }

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

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

Plan: 1 to add, 1 to change, 0 to destroy.