[terraform-ibm-icd-edb] Scope KMS policy to the exact KMS key

[ocofaigh]

The policy here can be updated to scope it to the exact KMS key. For an example of the syntax, see terraform-ibm-modules/terraform-ibm-cos#764

[arya-girish-k]

CC : @SarikaSinha

[arya-girish-k]

As per discussion with @ocofaigh , some more issues will be created related to this task. Hence un-assigning myself for now.

[ocofaigh]

#320