try to set default DNS server if it was not specified

[ShiChangkuo]

What this PR does / why we need it:
fixes #1074

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #xxx

Special notes for your reviewer:

Release note:

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccVpcSubnetV1_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccVpcSubnetV1_basic -timeout 360m -parallel 4
=== RUN   TestAccVpcSubnetV1_basic
=== PAUSE TestAccVpcSubnetV1_basic
=== CONT  TestAccVpcSubnetV1_basic
--- PASS: TestAccVpcSubnetV1_basic (85.90s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       85.959s