Adding target group level tagging

[snoozan]

PR o'clock

Description

Sometimes it's necessary to tag target groups at the target group level, rather than just inheriting from the parent ALB.

For example, if we have an ALB for all our QA services, the target groups underneath are tagged with what host group they are. This is useful for if you have heterogenous services running under the same LB. In our case, we have several dev instances that are tagged with their HostGroup : dev and Index: n for easy filtering on which target groups are available to stick an instance under. I've made these changes for our local fork, but since we'd like to keep upstream changes and avoid conflicts, I figured making this PR would be a good idea. Let me know if this is something you'd like to see in the module!

Example:

 "id": "arn:aws:elasticloadbalancing:us-west-2:794037559540:targetgroup/bar/2e899aec2bc4367c",
             "lambda_multi_value_headers_enabled": false,
             "name": "bar",
             "name_prefix": null,
             "port": 8080,
             "protocol": "HTTP",
             "proxy_protocol_v2": false,
             "slow_start": 100,
             "stickiness": [
               {
                 "cookie_duration": 86400,
                 "enabled": true,
                 "type": "lb_cookie"
               }
             ],
             "tags": {
               "Environment": "test",
               "GithubOrg": "terraform-aws-modules",
               "GithubRepo": "tf-aws-alb",
               "HostGroup": "Bar",
               "Name": "bar",
               "Workspace": "default"
             },
             "target_type": "instance",

Checklist

• terraform fmt and terraform validate both work from the root and examples/alb_test_fixture directories (look in CI for an example)
• Tests for the changes have been added and passing (for bug fixes/features)
• Test results are pasted in this PR (in lieu of CI)
• Docs have been added/updated (for bug fixes/features)
• Any breaking changes are noted in the description above

Testing notes

Test: https://gist.github.com/snoozan/9d6c494cdfdab8f8427fdb031bf957a8
The RSpec framework being used doesn't spec out tags at the target group level. This could be added to the library, but as of right now, no spec tests were written because of the lack of target group tags. See: k1LoW/awspec@a3e729e#diff-1acc82be899fcf02f9d1974a4850e3f8

[snoozan]

Not sure if I am supposed to set the AWS credentials in the PR: https://travis-ci.org/terraform-aws-modules/terraform-aws-alb/jobs/577922309

[snoozan]

@antonbabenko Have you had a chance to take a look at this?

[bryantbiggs]

this functionality has been covered under #151

[github-actions]

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