v6prefix auto generation and tests
tweaks based on review feedback, checked with tests
Merge pull request #230 from cloudedge/pull-req-master-7b2595345449549a947599bce19b7b2359708d68

https://github.com/crowbar/barclamp-network/pull/230

Merged by devtool for VictorLowther
Fix up auto v6prefix handling to work properly in prodction.
Expose the v6prefix attribute
Merge pull request #232 from VictorLowther/pull-req-master-b270781498ad8e7eff7e1b087b76903f225e98ee

https://github.com/crowbar/barclamp-network/pull/232

Merged by devtool for cloudedge