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.
```
5ece0f2d04