Refactor dhcp_database role along hte lines of the DNS database role.

It seems to give more consistent results.
Merge pull request #214 from VictorLowther/pull-req-master-7e4d4af3d6449b9d69d65e99a3e4e9c72ba47979

https://github.com/crowbar/barclamp-provisioner/pull/214

Merged by devtool for cloudedge