Clarify name filters for listing

[mderka]

In Dns interface, we should provide options for listing zone with a specified domain name only. However, this must be unique within project, so this operation would just become get. Similarly, when listing DNS records, one can specify type which has to be accompanied by a fully qualified domain name, and it turns into get. A question was sent to the service owners. These options should be either included if there is sense in such filters, or removed, if list becomes get (possibly with get being overloaded if appropriate).

@benvitale

[mderka]

Zone allow filter for dnsName with selector dnsName=www.example.com.. While name is unique within a project, dnsName does not need to be. I am adding an option for this.

[mderka]

DnsRecord listing allows filters for both dnsName and type. We already have them.