DNS server fixes and improvements

[andreasbell]

This PR adds a number of fixes and improvements to the DNS server, including:

• Adding configurable DNS suffix. This can be used to add an optional DNS suffix for example .nebula to all DNS records. This is useful for adding a common namespace for all nebula hosts, and ensuring that all host have a fully qualified domain name.
• Adding PTR records for all IP addresses. This makes it possible to do reverse DNS lookup and finding the host name from an IP address.
• Fixing broken TXT records. There seems to be a bug in the current implementation stopping the DNS server form sending out TXT records. This PR fixes this issue, and reduces the information in the TXT record to only include Name, Networks/Ips, Groups, and UnsafeNetworks
• Adding support for multiple IP addresses by providing one A/AAAA record for each IPv4/IPv6 address of the host.
• Adds DNS record for the host runng the DNS server.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @andreasbell to sign the Salesforce Inc. Contributor License Agreement.