[nat gateway] use unified network parameters

[ShiChangkuo]

• mark router_id as deprecated and use vpc_id instead
• mark internal_network_id as deprecated and use subnet_id instead
• mark network_id as deprecated and use subnet_id instead

What this PR does / why we need it:

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 TestAccNatGateway'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccNatGateway -timeout 360m -parallel 4
=== RUN   TestAccNatGatewayDataSource_basic
=== PAUSE TestAccNatGatewayDataSource_basic
=== RUN   TestAccNatGateway_basic
=== PAUSE TestAccNatGateway_basic
=== RUN   TestAccNatGateway_withEpsId
=== PAUSE TestAccNatGateway_withEpsId
=== CONT  TestAccNatGatewayDataSource_basic
=== CONT  TestAccNatGateway_withEpsId
=== CONT  TestAccNatGateway_basic
--- PASS: TestAccNatGateway_withEpsId (86.75s)
--- PASS: TestAccNatGatewayDataSource_basic (93.17s)
--- PASS: TestAccNatGateway_basic (103.19s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       103.234s

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccNatSnatRule_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccNatSnatRule_basic -timeout 360m -parallel 4
=== RUN   TestAccNatSnatRule_basic
=== PAUSE TestAccNatSnatRule_basic
=== CONT  TestAccNatSnatRule_basic
--- PASS: TestAccNatSnatRule_basic (110.75s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       110.804s

$ make testacc TEST='./huaweicloud' TESTARGS='-run TestAccNatGatewayDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccNatGatewayDataSource_basic -timeout 360m -parallel 4
=== RUN   TestAccNatGatewayDataSource_basic
=== PAUSE TestAccNatGatewayDataSource_basic
=== CONT  TestAccNatGatewayDataSource_basic
--- PASS: TestAccNatGatewayDataSource_basic (93.26s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       93.313s