Add the abbility to setup default zone with tenantid variable inside

Neutron has, in DEFAULT section in neutron.conf file, a key dns_domain which is a domain fallback.

Basically, if we create port with neutron, the default dns assigment will be myport.defaultzone.com.

With Designate Integration, that will create DNS record myport.defaultzone.com. A portIP

The problem is, defaultzone.com. can be created only inside one and only one tenant. So this feature works only for one tenant, because the others has not access to this zone.

One of the solution I propose, is to integrate tenantid variable into default zone. So basically, create <tenantid>.defaultzone.com.
on each tenants, to allow users to have their DNS record myport.<tenantid>.defaultzone.com. inside their personal zone.

To sumarize,

When the user do openstack port create --network public --dns-name myport port, a default record will be created at myport.<tenantid>.defaultzone.com.

So, when a user creates an instance with nova, or port with neutron, we want to create automatically dns record myport.<tenantid>.defaultzone.com. inside his own tenant. (A and AAAA fields)

Here is the code for now https://github.com/openstack/neutron/compare/master...gregoiremahe:patch/dns-integration