Fix issue with an instances label causing a ForceNew if omitted.
Also updates mistyped docs for the `opc_compute_security_list` resource.
```
$ make testacc TEST=./builtin/providers/opc TESTARGS="-run=TestAccOPCInstance_emptyLabel"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/21 09:57:48 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/opc -v -run=TestAccOPCInstance_emptyLabel -timeout 120m
=== RUN TestAccOPCInstance_emptyLabel
--- PASS: TestAccOPCInstance_emptyLabel (574.79s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/opc 574.835s
```