We were not setting all the values in the read Func. One other issue, we
were setting the *wrong* db_parameter_name value to state
```
make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSRDSClusterInstance_import'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSRDSClusterInstance_import -timeout 120m
=== RUN TestAccAWSRDSClusterInstance_importBasic
--- PASS: TestAccAWSRDSClusterInstance_importBasic (1201.80s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws
1201.814s
```
Usage of a helper function was assuming that an error would be returned
in a not found condition, when in fact a nil pointer was
returned.
Attached test crashes w/o fix, passes with it.
Fixes#5350
Refs #5418
Paul Stack