resource/aws_opsworks_stack: Use Default VPC when no VPC ID passed

[gdavison]

When no vpc_id is passed to aws_opsworks_stack, use the default VPC to match the behaviour of the AWS API.

Relates #26525
Relates #26666
Relates #23625

Output from acceptance testing:

$ make testacc PKG=opsworks TESTS=TestAccOpsWorksStack_

    acctest.go:696: skipping tests; AWS_DEFAULT_REGION (us-west-2) does not equal us-east-1
--- SKIP: TestAccOpsWorksStack_tagsAlternateRegion (1.02s)
--- PASS: TestAccOpsWorksStack_disappears (40.06s)
--- PASS: TestAccOpsWorksStack_noVPC_defaultAZ (40.93s)
--- PASS: TestAccOpsWorksStack_basic (41.47s)
--- PASS: TestAccOpsWorksStack_windows (56.94s)
--- PASS: TestAccOpsWorksStack_noVPC_basic (62.84s)
--- PASS: TestAccOpsWorksStack_tags (73.88s)
--- PASS: TestAccOpsWorksStack_allAttributes (74.94s)

$ AWS_DEFAULT_REGION=us-east-1 AWS_ALTERNATE_REGION=us-west-1 make testacc PKG=opsworks TESTS=TestAccOpsWorksStack_tagsAlternateRegion

--- PASS: TestAccOpsWorksStack_tagsAlternateRegion (95.42s)

[ewbankkit]

LGTM 🚀.

% make testacc TESTARGS='-run=TestAccOpsWorksStack_' PKG=opsworks ACCTEST_PARALLELISM=3
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/opsworks/... -v -count 1 -parallel 3  -run=TestAccOpsWorksStack_ -timeout 180m
=== RUN   TestAccOpsWorksStack_basic
=== PAUSE TestAccOpsWorksStack_basic
=== RUN   TestAccOpsWorksStack_disappears
=== PAUSE TestAccOpsWorksStack_disappears
=== RUN   TestAccOpsWorksStack_noVPC_basic
=== PAUSE TestAccOpsWorksStack_noVPC_basic
=== RUN   TestAccOpsWorksStack_noVPC_defaultAZ
=== PAUSE TestAccOpsWorksStack_noVPC_defaultAZ
=== RUN   TestAccOpsWorksStack_tags
=== PAUSE TestAccOpsWorksStack_tags
=== RUN   TestAccOpsWorksStack_tagsAlternateRegion
=== PAUSE TestAccOpsWorksStack_tagsAlternateRegion
=== RUN   TestAccOpsWorksStack_allAttributes
=== PAUSE TestAccOpsWorksStack_allAttributes
=== RUN   TestAccOpsWorksStack_windows
=== PAUSE TestAccOpsWorksStack_windows
=== CONT  TestAccOpsWorksStack_basic
=== CONT  TestAccOpsWorksStack_tags
=== CONT  TestAccOpsWorksStack_allAttributes
--- PASS: TestAccOpsWorksStack_basic (38.52s)
=== CONT  TestAccOpsWorksStack_windows
--- PASS: TestAccOpsWorksStack_allAttributes (68.63s)
=== CONT  TestAccOpsWorksStack_tagsAlternateRegion
    acctest.go:696: skipping tests; AWS_DEFAULT_REGION (us-west-2) does not equal us-east-1
--- SKIP: TestAccOpsWorksStack_tagsAlternateRegion (0.31s)
=== CONT  TestAccOpsWorksStack_noVPC_basic
--- PASS: TestAccOpsWorksStack_tags (70.93s)
=== CONT  TestAccOpsWorksStack_noVPC_defaultAZ
--- PASS: TestAccOpsWorksStack_windows (59.21s)
=== CONT  TestAccOpsWorksStack_disappears
--- PASS: TestAccOpsWorksStack_noVPC_defaultAZ (40.84s)
--- PASS: TestAccOpsWorksStack_disappears (31.09s)
--- PASS: TestAccOpsWorksStack_noVPC_basic (62.13s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/opsworks	134.874s

[github-actions]

This functionality has been released in v4.30.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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