Correctly sets the attribute `ip_address` in the `opc_compute_ip_address_reservation` resource.
Also updates documentation for the `ip_address_pool` attribute.
```
$ make testacc TEST=./builtin/providers/opc TESTARGS="-run=TestAccOPCIPAddressReservation_Basic"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 10:15:53 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/opc -v -run=TestAccOPCIPAddressReservation_Basic -timeout 120m
=== RUN TestAccOPCIPAddressReservation_Basic
--- PASS: TestAccOPCIPAddressReservation_Basic (22.60s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/opc 22.604s
```