ovn-northd: Add logical flows to support native DNS

ovn-northd: Add logical flows to support native DNS

OVN implements native DNS resolution which can be used to resolve the
internal DNS names belonging to a logical datapath.

To support this, a new table 'DNS' is added in the NB DB. A new column
'dns_records' is added in 'Logical_Switch' table which references to the
'DNS' table.

Following flows are added for each logical switch if configured with
DNS records in the 'dns_records' column
 - A logical flow in DNS_LOOKUP stage which uses the action 'dns_lookup'
   to transform the DNS query to DNS reply packet and advances
   to the next stage - DNS_RESPONSE.

 - A logical flow in DNS_RESPONSE stage which implements the DNS responder
   by sending the DNS reply from previous stage back to the inport.

Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Acked-by: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>