Define behavior for a KMS-Keyring when given an generator which is an alias

[MatthewBennington]

Presently the spec OnEncrypt section of the spec says that the keyring should use a "client that calls the AWS region specified in the generator ARN".

In cases where the keyring's generator is an alias (e.g. "alias/MyCryptoKey"), there is no way to determine the region to make call is.

[seebees]

This is also true for the keyIds as well. Notes are in the PR, but this is not a generator specific problem

[acioc]

Closing in favor of #165