provider/aws: Don't set DBName on aws_db_instance from snapshot

[phinze]

It turns out if you're trying to write a config to conditionally restore
an instance from a snapshot, you end up painted into a corner with the
combination of snapshot_identifier and name.

You need name to be specified for DBs you're creating, but when
snapshot_identifier is populated you need it to be blank or else the
AWS API throws an error.

The logical next step is to drop a ternary in:

resource "aws_db_instance" "foo" {
 # ...
 snapshot_identifier = "${var.snap}"
 name = "${var.snap != "" ? "" : var.dbname}"
}

BUT the above config will replace the DB on subsequent runs as the
config basically has name = "" which will trigger a ForceNew diff once
the name is populated from the snapshot restore.

SO we can get a workable solution by actively avoiding populating
DBName when we're using one of the engines the docs explicitly mention.

It does not look like there are any tests covering snapshot_identifer,
so I'll subject this to some manual tests and follow up with some
results.

[phinze]

Manually confirmed that using the following config:

resource "aws_db_instance" "test" {
  identifier                = "snapshot-identifier-test"
  engine                    = "postgres"
  db_subnet_group_name      = "..."
  username                  = "..."
  password                  = "..."
  instance_class            = "db.t2.small"
  vpc_security_group_ids    = ["..."]
  name                      = "(dbname from snapshot)"
  snapshot_identifier       = "( a snapshot )"
}

Master yields:

Error creating DB Instance: InvalidParameterValue: DBName must be null when Restoring for this Engine.

And this branch works correctly, with the subsequent plan being empty provided that the name parameter is set to the same name as the database in the snapshot.

[grubernaut]

LGTM, Thanks!!

[ghost]

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