provider/aws: aws_db_instance restored from snapshot had problem with (#11050)
provider/aws: aws_db_instance restored from snapshot had problem with (#11050)

subnet_group

Fixes #11024

A change was introduced in 0.8.2 that allows db_instances to change
their db_subnet_group. Unfortunately, this caused an issue for
db_instances that were being restored from snapshot. The restore from
snapshot part of create calls the Update func whereas a normal create
calls the Read func

When calling the Update func, the db_instance was trying to go through a
db_subnet_group_name change and was failing for the following reason:

```
InvalidVPCNetworkStateFault: You cannot move DB instance _rds_instance_name_ to subnet group _subnet_group_name_. The specified DB subnet group and DB instance are in the same VPC. Choose a DB subnet group in different VPC than the specified DB instance and try again.
```
ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws