Added public IP address option for nameservers

For corporate, private clouds, it is desirable to resolve addresses in the cloud managed DNS servers.

The default proposal will not allow integration with all corporate DNS servers.

Eg. When configuring Microsoft DNS with a conditional forwarder, it will resolve the **private** nameserver names from the Cloud DNS server as configured automatically by this barclamp even if a **public** address is specified (to the Microsoft DNS server) and configured on the admin servers.

To allow corporate DNS integration, the name servers, as configured by this barclamp, need to use alternative **public** IP addresses.

To use this change:

Add public IP addresses for all DNS servers as custom DNS entry(ies) in DNS proposal.
Update nameservers to use the public DNS IP address('s).
Configure relevant corporate DNS server to resolve
In case you're more interested, there's a more idiomatic Ruby way of writing this:

    if cluster_zone[:namerservers].empty?