Reconsider how data center id is handled

[mdb]

In current implementation, a data center's Id() is the data center's .DataCenterID.

However, in Akamai, a DC DataCenterID is not universally unique across a GTM domain; it is unique only within a GTM domain. Therefore, it may be worth considering making the Terraform data center resource's Id() something unique and domain-namespaced such as ${gtm_property}:${.DataCenterID} to avoid resource collision across properties.

For example, consider the following:

1. User creates a .tf creating a DC in some-domain
2. User executes terraform plan && terraform apply, in effect creating DC 123 in some-domain
3. User edits the .tf to target some-other-domain
4. Given the tfstate from step 2, Terraform may modify a DC 123 rather than create a fresh DC in the event that a DC 123 already exists in some-other-domain.