Fix credential regression

[hmlanigan]

Description

Juju's CreateModel and ModelInfo provide different data for the credential used, the former returns a CloudCredential as a string, the latter returns a CloudCredentialTag as a tag. When they are both the same data. This led to different data being written as a model's credential in the terraform model resource Read and Create methods.

The second step is to reduce the juju concerns within the provider pieces of code by updating CreateModelResponse to contain what is required in the format required, rather than a struct from the juju package. This should be done more as we can.

Due to privacy concerns around cloud credentials, creating an acceptance test for this change is problematic and not done at this time. Perhaps in the future a private GitHub action and environment can be created to test.

Fixes: #310

Type of change

• Bug fix (non-breaking change which fixes an issue)

QA steps

terraform {
  required_providers {
    juju = {
      version = ">= 0.9.0"
      source  = "juju/juju"
    }
  }
}

provider "juju" {
}

resource "juju_model" "testmodel" {
  name = "applicationtest"
}

resource "juju_model" "bugmodel" {
  name = "credentialtest"
  cloud {
    name = "aws"
    region = "us-east-1"
  }
  credential = juju_credential.test-creds.name
}

resource "juju_credential" "test-creds" {
  name = "test-creds"
  cloud  {
    name = "aws"
  }
  auth_type = "access-key"
  attributes = {
    access-key = <add-your-details>
    secret-key = <add-your-details>
  }
}

resource "juju_application" "ubuntu" {
  model = juju_model.bugmodel.name
  charm {
    name     = "ubuntu"
    series   = "jammy"
  }
}

resource "juju_application" "application_example" {
  name  = "app-example"
  model = juju_model.testmodel.name
  charm {
    name     = "ubuntu"
    series   = "jammy"
  }
}

Run with the above plan

$ juju bootstrap localhost terraform
$ juju add-cloud aws --controller terraform --force
$ terraform init && terraform plan && terraform apply
$ terraform plan && terraform apply

Additional notes

JUJU-4628

[cderici]

This LGTM, QA went well 👍

Totally agree about the thicker boundary/separation with the Juju structs.