feat(bgp-settings): add keepalive interval

[aorfanos]

Recent runs with this module display the following message

bgp {
    keepalive_interval = 0 -> 20
    # (3 unchanged attributes hidden)
}

This change sets the default as per the docs and allows for customization.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

For more information, open the CLA check for this pull request.

[comment-bot-dev]

Thanks for the PR! 🚀 Unfortunately it looks like some of our CI checks failed. See the Contributing Guide for details.

• ⚠️check_documentation The documentation needs to be regenerated. Please run make generate_docs.

  Checking for documentation generation
  diff -r '--exclude=.terraform' '--exclude=.kitchen' '--exclude=.git' '--exclude=autogen' '--exclude=*.tfvars' /workspace/README.md /tmp/tmp.DdeXpiRP0Y/generate_docs/workspace/README.md
  56a57
  > | router\_bgp\_keepalive | BGP keepalive message. Defaults to 20s if not set. Must be an integer between 20 and 60 that specifies the number of seconds for the interval. | `string` | `"20"` | no |
  Error: Documentation generation has not been run, please run the
  'make docker_generate_docs' command and commit the above changes.

• ⚠️check_terraform Failed Terraform check. More details below.

  
  Running terraform fmt
  Running terraform validate
  terraform_validate . 
  ╷
  │

Error: Unsupported argument │

│

on main.tf line 42, in resource "google_compute_router" "router": │

42:
keepalive = var.router_bgp_keepalive │

│

An argument named "keepalive" is not expected here. ╵ terraform_validate ./examples/advanced ╷ │

Error: Unsupported argument │

│

on ../../main.tf line 42, in resource "google_compute_router" "router": │

42:
keepalive = var.router_bgp_keepalive │

│

An argument named "keepalive" is not expected here. ╵ terraform_validate ./examples/basic ╷ │

Error: Unsupported argument │

│

on ../../main.tf line 42, in resource "google_compute_router" "router": │

42:
keepalive = var.router_bgp_keepalive │

│

An argument named "keepalive" is not expected here. ╵ terraform_validate ./examples/nat_with_compute_engine Success! The configuration is valid. terraform_validate ./examples/nat_with_gke Success! The configuration is valid. terraform_validate ./test/fixtures/advanced ╷ │

Error: Unsupported argument │

│

on ../../../main.tf line 42, in resource "google_compute_router" "router": │

42:
keepalive = var.router_bgp_keepalive │

│

An argument named "keepalive" is not expected here. ╵ terraform_validate ./test/fixtures/basic ╷ │

Error: Unsupported argument │

│

on ../../../main.tf line 42, in resource "google_compute_router" "router": │

42:
keepalive = var.router_bgp_keepalive │

│

An argument named "keepalive" is not expected here. ╵ terraform_validate ./test/fixtures/subnetworks ╷ │

Error: Unsupported argument │

│

on ../../../main.tf line 42, in resource "google_compute_router" "router": │

42:
keepalive = var.router_bgp_keepalive │

│

An argument named "keepalive" is not expected here. ╵ terraform_validate ./test/setup Success! The configuration is valid.