title: Use terrform to update a KMS Key Policy
body: <p>Does anyone know how I would get Terraform to UPDATE an existing KMS Key policy?</p>
  
  <p>I already have the KMS Key(s) created but I have a temporary IAM role that needs to use an existing KMS key. I would like to be able to add this "new" IAM role to the existing KMS Key policy. I can see that this is possible using the AWS console but I can't see how do do this in Terraform.</p>
  
  <p>I would imagine that this is also possible using the AWS CLI tools but I haven't investigated that as I am building an environment in Terraform and would like to keep it all in the one place.</p>
  
  <p>I should point out that the current KMS key is used to encrypt S3 uploads and downloads and various IAM users and roles already have access to the current key so creating a new key would just invert the issue for those already accessing the buckets.</p>