Add arn attribute to aws_ssm_parameter resource and datasource

[bflad]

Not provided by DescribeParameters API, so built manually. References:

• http://docs.aws.amazon.com/sdk-for-go/api/service/ssm/#ParameterMetadata
• http://docs.aws.amazon.com/general/latest/gr/aws-arns-and-namespaces.html#arn-syntax-ssm

Closes #2041

make testacc TESTARGS='-run=TestAccAwsSsmParameterDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccAwsSsmParameterDataSource_basic -timeout 120m
?   	github.com/terraform-providers/terraform-provider-aws	[no test files]
=== RUN   TestAccAwsSsmParameterDataSource_basic
--- PASS: TestAccAwsSsmParameterDataSource_basic (14.16s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	14.193s

make testacc TESTARGS='-run=TestAccAWSSSMParameter_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccAWSSSMParameter_basic -timeout 120m
?   	github.com/terraform-providers/terraform-provider-aws	[no test files]
=== RUN   TestAccAWSSSMParameter_basic
--- PASS: TestAccAWSSSMParameter_basic (12.16s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	12.196s

[radeksimko]

LGTM, thanks. 👍 🚀

[ghost]

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