internalprovider: fix inconsistent datacenter references

[dkropachev]

Fixes: https://github.com/scylladb/terraform-provider-scylladbcloud/issues/116 https://github.com/scylladb/terraform-provider-scylladbcloud/issues/130

Tested:

main.tf:

resource "random_string" "postfix" {
  length           = 8
  special          = false
}

resource "scylladbcloud_cluster" "aws" {
  name       = "AWS-statuspage-test2-${random_string.postfix.result}"
  cloud      = "AWS"
  region     = "us-east-1"
  node_count = 3
  node_type  = "t3.micro"
  cidr_block = "172.31.0.0/24"
  enable_dns = true
}

data "scylladbcloud_cql_auth" "example" {
  cluster_id = scylladbcloud_cluster.aws.id
}

Steps for upgrade scenario:

1. Run tf init; tf apply, on current version (1.5.0) of terraform provider
2. Upgrade tf provider and run tf apply

Steps for create scenario:

1. Upgrade tf provider and run tf apply

Steps for create scenario with datacenter:

1. Change cql_auth definition:

   data "scylladbcloud_cql_auth" "example" {
   cluster_id = scylladbcloud_cluster.aws.id
   datacenter = "AWS_US_EAST_1"
   }

2. Upgrade tf provider and run tf apply