ovn-controller: Omit tracking external_ids columns

ovn-controller: Omit tracking external_ids columns

Running the command "ovn-nbctl set logical_switch_port foo external_ids:foo=bar"
results in the incremetal processing engine to recompute the flows on the
chassis where the logical port 'foo' is claimed.

This patch avoids this unnecessary recomputation by omitting the tracking of
external_ids column of all the Southbound DB tables except DNS, Chassis
and Datapath_Binding tables. ovn-controller is refering to the external_ids
column of these tables.

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