Ensure valid datapoints_to_alarm value

We're seeing AWS reject requests for this:
```
datapoints_to_alarm:      "0"
```

It turns out it must be >= 1, and when omitted, CloudWatch Alarms does the right thing, but Terraform wants to default this to "0" still:

hashicorp/terraform-provider-aws#4350 (problem report)
hashicorp/terraform-provider-aws#5095 (validation to 1)

main.tf

@@ -56,6 +56,7 @@ resource "aws_cloudwatch_metric_alarm" "status_check_failed_instance_alarm_reboo
   period              = "${var.failed_instance_checks_period}"
   statistic           = "Minimum"
   threshold           = "0"
+  datapoints_to_alarm = "1"
   unit                = "Count"
 
   dimensions {
@@ -77,6 +78,7 @@ resource "aws_cloudwatch_metric_alarm" "status_check_failed_instance_alarm_ticke
   period              = "${var.failed_instance_checks_period}"
   statistic           = "Minimum"
   threshold           = "0"
+  datapoints_to_alarm = "1"
   unit                = "Count"
 
   dimensions {