support RDS encryption at rest

[ryanking]

https://aws.amazon.com/about-aws/whats-new/2015/01/06/amazon-rds-encryption-with-kms-mysql-postgresql/

[catsby]

Looks like we'd need to add support for StorageEncrypted and KmsKeyId into resource_aws_db_instance.go

@ryanking is this something you want to take a stab at? If not, I can try and get to it in the near future, but I can't make promises as to when. I agree that it's a thing we should support 😄

[josh-padnick]

+1 for this feature. It's especially important because RDS will only let you set this option at RDS Launch Time. You can't modify an existing RDS instance to support this.

[phinze]

Done in #1041 👍

[ghost]

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