Use aws_secretsmanager_secret prefix instead of hard name to prevent conflicts

[jim80net]

Secrets manager takes 30 days to delete a key, which doesn't fit into a terraform workflow if ever a secret needs deleting and recreation.

Use name prefix for aws_secretsmanager_secret instead to prevent the following error:

module.datadog.aws_secretsmanager_secret.datadog_api_key: Creating...
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [10s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [20s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [30s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [40s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [50s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [1m0s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [1m10s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [1m20s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [1m30s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [1m40s elapsed]
module.datadog.aws_secretsmanager_secret.datadog_api_key: Still creating... [1m50s elapsed]

Error: error creating Secrets Manager Secret: InvalidRequestException: You can't create this secret because a secret with this name is already scheduled for deletion.

  on .terraform/modules/datadog/terraform-aws-datadog-1.0.0/logs_monitoring.tf line 19, in resource "aws_secretsmanager_secret" "datadog_api_key":
  19: resource aws_secretsmanager_secret "datadog_api_key" {