provider/aws: Add 'aws_kms_ciphertext' data source

[ewbankkit]

Inspired by #8774, add an aws_kms_ciphertext data source that allows you to encrypt plaintext into ciphertext by using an AWS KMS customer master key.

Acceptance tests:

make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccDataSourceAwsKmsCiphertext_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 18:16:07 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccDataSourceAwsKmsCiphertext_ -timeout 120m
=== RUN   TestAccDataSourceAwsKmsCiphertext_basic
--- PASS: TestAccDataSourceAwsKmsCiphertext_basic (42.76s)
=== RUN   TestAccDataSourceAwsKmsCiphertext_validate
--- PASS: TestAccDataSourceAwsKmsCiphertext_validate (44.72s)
=== RUN   TestAccDataSourceAwsKmsCiphertext_validate_withContext
--- PASS: TestAccDataSourceAwsKmsCiphertext_validate_withContext (44.60s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	132.087s

[coen-hyde]

Awesome @ewbankkit. Linking #6133

[ewbankkit]

Rebase and simplify creation of ID.

[stack72]

This is a great addition! Thanks so much @ewbankkit :)

% make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccDataSourceAwsKmsCiphertext_'                                              ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/22 15:41:48 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccDataSourceAwsKmsCiphertext_ -timeout 120m
=== RUN   TestAccDataSourceAwsKmsCiphertext_basic
--- PASS: TestAccDataSourceAwsKmsCiphertext_basic (61.98s)
=== RUN   TestAccDataSourceAwsKmsCiphertext_validate
--- PASS: TestAccDataSourceAwsKmsCiphertext_validate (63.52s)
=== RUN   TestAccDataSourceAwsKmsCiphertext_validate_withContext
--- PASS: TestAccDataSourceAwsKmsCiphertext_validate_withContext (63.63s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	189.171s

[ghost]

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