Horizon missing fields of Neutron ML2 dns extension

Since now designate its fully API integrated with nova and neutron in order to provide autoregistrarion on DNS of fixed ips and floating ips, the Neutron ML2 dns extension was added in order to provide this functionality.

But now the users can only handle this via CLI cause horizon is missing the dns fields/functionality on the launch and instance and the floating ip create forms on Horizon

Missing fields and extension documentation on the following link: http://docs.openstack.org/mitaka/networking-guide/config-dns-int.html

Link to the extension code: https://github.com/openstack/neutron/blob/master/neutron/plugins/ml2/extensions/dns_integration.py

Horizon should be able to, if extension enabled:

- Add dns_name field (allow user input) when launching an instance to pass it trought the fixed ip port creation. If none is provided it should auto-assign it as $VM_HOSTNAME + dns_domain field attribute from the network if it has it. If selecting an existing port it should override the attribute of the port if exists.
- On network creation screen add the dns_domain field (user input) as an attribute to the network
- On floating ip creation screen add the dns_domain field and the dns_name field (both user input) to assign to the floating ip. If none is provided it should auto-assing $FLOATING-IP-NAME + dns_domain attribute from the external network if exists

A backport to mitaka will be really appreciated :)