aws_db_subnet_group: "Default" is not allowed as "name"

[xescugc]

General information:

• Operating System:
• Terracognita version / tag:
• Did you build Terracognita from sources or did you use the Docker image:

Describe the bug:

When importing some aws_db_subnet_group we may get

╷
│ Error: "Default" is not allowed as "name"
│ 
│   with module.tf.aws_db_subnet_group.default,
│   on module-tf/rds_relational_database.tf line 98, in resource "aws_db_subnet_group" "default":
│   98:   name        = aws_vpc.cycloid_test.instance_tenancy
│ 
╵

Those are the resources managed by AWS so we should ignore them and not import/write them.

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Additional context

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