Add domain attribute to cluster_rosa_classic resource

[jjulien]

The dns.base_domain attribute is immediately available from the API when creating a new cluster, and can be useful in further configuration of resources within the same terraform run.

This value can be queried using rosa describe cluster -c id. The value from the ROSA CLI is always [name].[dns.base_domain], which is why the domain attribute prepends the cluster name in this proposed feature.

An example use would be configuring an OpenID IdP using the domain attribute for the callback URL.

This creates a nice workflow where the cluster creation can begin in the background (required already because of the OIDC Provider dependency), and the ocm_identity_provider of type OpenID can be created in the same terraform run, which will trigger the wait for cluster install completion loop on ocm_identity_provider.

The result is a single terraform run for a ROSA cluster build, that does not exit until the cluster is ready for use, with SSO login enabled.

[bardielle]

@jjulien This repository is deprecated, please open the same PR in the new repository - https://github.com/terraform-redhat/terraform-provider-ocm