Implement multiple versions for regional instance groups

[oceyral]

Related to Issue #1793.

This is essentially a port of PR #1499 for regional instance groups.

It was tested with the following :

## Instance group
resource "google_compute_region_instance_group_manager" "smtpin" {
  name = "smin00-${data.google_client_config.current.region}"
  region = "${data.google_client_config.current.region}"
  base_instance_name = "${var.mailjet_env}-li-smin00"
  target_size = "${data.consul_key_prefix.smtpin.var.scale}"
  update_strategy = "ROLLING_UPDATE"
  target_pools = ["${google_compute_target_pool.smtpin.self_link}"]

  rolling_update_policy {
    type = "PROACTIVE"
    minimal_action = "REPLACE"
    max_surge_fixed = 3
  }

  version {
    name = "smtpin-current"
    instance_template = "${google_compute_instance_template.smtpin-current.self_link}"
  }
  version {
    name = "smtpin-next"
    instance_template = "${google_compute_instance_template.smtpin-next.self_link}"
    target_size {
      fixed = 1
    }
  }
}

testacc passes when limited to the resource in question :

TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -timeout 120m -run 'RegionInstanceGroupManager'
?       github.com/terraform-providers/terraform-provider-google        [no test files]
=== RUN   TestAccRegionInstanceGroupManager_basic
=== PAUSE TestAccRegionInstanceGroupManager_basic
=== RUN   TestAccRegionInstanceGroupManager_targetSizeZero
=== PAUSE TestAccRegionInstanceGroupManager_targetSizeZero
=== RUN   TestAccRegionInstanceGroupManager_update
=== PAUSE TestAccRegionInstanceGroupManager_update
=== RUN   TestAccRegionInstanceGroupManager_updateLifecycle
=== PAUSE TestAccRegionInstanceGroupManager_updateLifecycle
=== RUN   TestAccRegionInstanceGroupManager_updateStrategy
=== PAUSE TestAccRegionInstanceGroupManager_updateStrategy
=== RUN   TestAccRegionInstanceGroupManager_rollingUpdatePolicy
=== PAUSE TestAccRegionInstanceGroupManager_rollingUpdatePolicy
=== RUN   TestAccRegionInstanceGroupManager_separateRegions
=== PAUSE TestAccRegionInstanceGroupManager_separateRegions
=== RUN   TestAccRegionInstanceGroupManager_versions
=== PAUSE TestAccRegionInstanceGroupManager_versions
=== RUN   TestAccRegionInstanceGroupManager_autoHealingPolicies
=== PAUSE TestAccRegionInstanceGroupManager_autoHealingPolicies
=== RUN   TestAccRegionInstanceGroupManager_distributionPolicy
=== PAUSE TestAccRegionInstanceGroupManager_distributionPolicy
=== CONT  TestAccRegionInstanceGroupManager_basic
=== CONT  TestAccRegionInstanceGroupManager_separateRegions
=== CONT  TestAccRegionInstanceGroupManager_updateStrategy
=== CONT  TestAccRegionInstanceGroupManager_updateLifecycle
=== CONT  TestAccRegionInstanceGroupManager_rollingUpdatePolicy
=== CONT  TestAccRegionInstanceGroupManager_distributionPolicy
=== CONT  TestAccRegionInstanceGroupManager_autoHealingPolicies
=== CONT  TestAccRegionInstanceGroupManager_versions
--- PASS: TestAccRegionInstanceGroupManager_basic (176.24s)
=== CONT  TestAccRegionInstanceGroupManager_update
--- PASS: TestAccRegionInstanceGroupManager_updateLifecycle (211.79s)
=== CONT  TestAccRegionInstanceGroupManager_targetSizeZero
--- PASS: TestAccRegionInstanceGroupManager_updateStrategy (247.28s)
--- PASS: TestAccRegionInstanceGroupManager_targetSizeZero (49.35s)
--- PASS: TestAccRegionInstanceGroupManager_versions (300.22s)
--- PASS: TestAccRegionInstanceGroupManager_rollingUpdatePolicy (305.20s)
--- PASS: TestAccRegionInstanceGroupManager_separateRegions (309.69s)
--- PASS: TestAccRegionInstanceGroupManager_distributionPolicy (370.74s)
--- PASS: TestAccRegionInstanceGroupManager_autoHealingPolicies (373.53s)
--- PASS: TestAccRegionInstanceGroupManager_update (215.24s)
PASS
ok      github.com/terraform-providers/terraform-provider-google/google 391.495s
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-google/scripts        0.009s [no tests to run]

[nat-henderson]

I'm comfortable with this, great. I'm running the tests now. If you could take a second look at that comment I flagged I'd appreciate it - I'll merge this as soon as I hear back from you.

[nat-henderson]

This long parenthetical doesn't read very fluently - can you take another look at this comment?

[oceyral]

I updated the comment, is it better now ?

[nat-henderson]

Tests all pass on our end as well. :)

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. If you feel I made an error 🤖 🙉 , please reach out to my human friends 👉 hashibot-feedback@hashicorp.com. Thanks!