Fix on_proposed to handle networks that don't have ranges.

It is permissible to declare networks without ranges if you just want
IPv6 address auto allocation to handle assigning addresses.
Merge pull request #233 from VictorLowther/pull-req-master-813ddc976a035cc6cb8e2f84507fac132aba7c83

https://github.com/crowbar/barclamp-network/pull/233

Merged by devtool for cloudedge