Add task_parameters parameter to aws_ssm_maintenance_window_task resource

[joelthompson]

This adds support for task_parameters to the aws_ssm_maintenance_window_task resource.

The ssm:RegisterTaskWithMaintenanceWindow AWS API method includes a TaskParameters parameters, which isn't currently supported by TF. This adds support that parameter.

Additionally, the docs had an incorrect name for the aws_ssm_maintenance_window's allow_unregistered_targets parameter (it was actually allow_unassociated_targets in the TF resource and the AWS API), so updating the doc to correct the name for that.

[stack72]

This LGTM! Thanks for this and also for picking up the error in our docs

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSSSMMaintenanceWindowTask'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/06/06 14:05:52 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSSSMMaintenanceWindowTask -timeout 120m
=== RUN   TestAccAWSSSMMaintenanceWindowTask_basic
--- PASS: TestAccAWSSSMMaintenanceWindowTask_basic (205.53s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	205.554s

Paul

[joelthompson]

Thanks for responding so quickly! :)

[ghost]

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