tests/resource/aws_db_subnet_group: Use aws_availability_zones data source to make testing region agnostic
tests/resource/aws_db_subnet_group: Use aws_availability_zones data source to make testing region agnostic

Output from acceptance testing in AWS Commercial:

```
--- PASS: TestAccAWSDBSubnetGroup_namePrefix (30.36s)
--- PASS: TestAccAWSDBSubnetGroup_generatedName (30.56s)
--- PASS: TestAccAWSDBSubnetGroup_basic (30.60s)
--- PASS: TestAccAWSDBSubnetGroup_importBasic (30.82s)
--- PASS: TestAccAWSDBSubnetGroup_withUndocumentedCharacters (32.38s)
--- PASS: TestAccAWSDBSubnetGroup_updateDescription (48.99s)
```

Output from acceptance testing in AWS GovCloud (US):

```
--- PASS: TestAccAWSDBSubnetGroup_basic (33.86s)
--- PASS: TestAccAWSDBSubnetGroup_namePrefix (34.13s)
--- PASS: TestAccAWSDBSubnetGroup_generatedName (34.13s)
--- PASS: TestAccAWSDBSubnetGroup_importBasic (34.71s)
--- PASS: TestAccAWSDBSubnetGroup_withUndocumentedCharacters (36.21s)
--- PASS: TestAccAWSDBSubnetGroup_updateDescription (55.57s)
```

terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws