Prompt for nameserver IP address in dnszone-add

Prompt for nameserver IP address in interactive mode of dnszone-add.

Add a corresponding field to dnszone creation dialog in the web UI.

This parameter is required if and only if:
* New zone is a forward zone
* Nameserver is defined inside the new zone

Add a new unit test to cover this functionality.

https://fedorahosted.org/freeipa/ticket/3603