add target_group_arn_suffixes output

[jeff-everett]

This is helpful in adding cloud watch monitor(s) for a given number of healthy instances:

From the module invocation, where we specify the list of target groups:
target_groups = "${list (map ("name", "${var.app1}", "backend_protocol", "HTTP", "backend_port", "8080" ), (map ("name", "${var.app2}", "backend_protocol", "HTTP", "backend_port", "80" ) ) )}"

Cloudwatch usage:
`resource "aws_cloudwatch_metric_alarm" "app1_unhealthy_hosts" {
alarm_name = "${var.app1}-unhealthy-host"
comparison_operator = "LessThanThreshold"
datapoints_to_alarm = "3"
evaluation_periods = "5"
metric_name = "HealthyHostCount"
namespace = "AWS/ApplicationELB"
period = "60"
statistic = "Maximum"
threshold = "2"

dimensions {
TargetGroup = "${element(module.alb.target_group_arn_suffixes, 0)}"
LoadBalancer = "${module.alb.load_balancer_arn_suffix}"
}

alarm_description = "${var.environment} ${var.service} ${var.app1} instance(s) failing health checks."

alarm_actions = [
"${module.notify_slack.this_slack_topic_arn}",
"${aws_sns_topic.pagerduty.arn}"
]

ok_actions = [
"${module.notify_slack.this_slack_topic_arn}",
"${aws_sns_topic.pagerduty.arn}"
]
}`

[brandonjbjelland]

Hey @jeff-everett 👋 Funny seeing you here, former teamy.

Everything looks good with your change. Thanks for the contribution and I'll cut a release with it shortly.

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