provider/aws: Change Redshift Cluster cluster_type to be computed

[stack72]

It will be based off the number of nodes. Can only be multi-node (>1 node) or single-node (nodes=1)

As requested in #5235

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRedshiftCluster' 2>~/tf.log
==> Checking that code complies with gofmt requirements...
/Users/stacko/Code/go/bin/stringer
GO15VENDOREXPERIMENT=1 go generate $(GO15VENDOREXPERIMENT=1 go list ./... | grep -v /vendor/)
TF_ACC=1 GO15VENDOREXPERIMENT=1 go test ./builtin/providers/aws -v -run=TestAccAWSRedshiftCluster -timeout 120m
=== RUN   TestAccAWSRedshiftCluster_basic
--- PASS: TestAccAWSRedshiftCluster_basic (598.03s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    598.047s

[jen20]

LGTM. Thanks @stack72! Fixes #5235.

[ghost]

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