When using load balancer rules on an IP associated with a network
instead of a vpc, the network field can be omitted and inferred from the
IP. Filling this into state on read causes a spurious diff.
The openfirewall flag defaults to true when used on a network IP.
Implicit resource creation doesn't fit the terraform model, so we
disable it.
Also added a test which shows arguments that can be changed without
creating a new resource.
Sander van Harmelen