Added private_dns_name to network_interface

[cosimomeli]

I added the private_dns_name field to the aws_network_interface resource as I needed it.
Changed test cases to add DNS hostnames in VPCs.

TF_ACC=1 go test -v gitb.com/terraform-providers/terraform-provider-aws/aws -timeout=7200s -run "TestAccAWSENI_.*"
=== RUN   TestAccAWSENI_importBasic
--- PASS: TestAccAWSENI_importBasic (68.99s)
=== RUN   TestAccAWSENI_basic
--- PASS: TestAccAWSENI_basic (65.42s)
=== RUN   TestAccAWSENI_updatedDescription
--- PASS: TestAccAWSENI_updatedDescription (106.23s)
=== RUN   TestAccAWSENI_attached
--- PASS: TestAccAWSENI_attached (296.42s)
=== RUN   TestAccAWSENI_ignoreExternalAttachment
--- PASS: TestAccAWSENI_ignoreExternalAttachment (174.64s)
=== RUN   TestAccAWSENI_sourceDestCheck
--- PASS: TestAccAWSENI_sourceDestCheck (58.91s)
=== RUN   TestAccAWSENI_computedIPs
--- PASS: TestAccAWSENI_computedIPs (64.46s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	835.092s

[Ninir]

Hey @cosimomeli

This looks very good to me for a first contribution! 👍

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSENI_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSENI_ -timeout 120m
=== RUN   TestAccAWSENI_importBasic
--- PASS: TestAccAWSENI_importBasic (157.48s)
=== RUN   TestAccAWSENI_basic
--- PASS: TestAccAWSENI_basic (160.02s)
=== RUN   TestAccAWSENI_updatedDescription
--- PASS: TestAccAWSENI_updatedDescription (247.85s)
=== RUN   TestAccAWSENI_attached
--- PASS: TestAccAWSENI_attached (427.12s)
=== RUN   TestAccAWSENI_ignoreExternalAttachment
--- PASS: TestAccAWSENI_ignoreExternalAttachment (287.33s)
=== RUN   TestAccAWSENI_sourceDestCheck
--- PASS: TestAccAWSENI_sourceDestCheck (122.42s)
=== RUN   TestAccAWSENI_computedIPs
--- PASS: TestAccAWSENI_computedIPs (120.85s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	1523.102s

Thanks a lot!

[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. Thanks!