Since [1], any "Logical_Router_Port" has a key in "external_ids" to
identify if it is an external gateway or not. This key is
"neutron:is_ext_gw". This patch makes use of this key to identify
them in the ``get_lrouter_gw_ports`` method.
This implementation will be useful when the "Logical_Router_Port"
would be bound to an external tunnelled networks. These ports won't
be bound to a "Gateway_Chassis" and the previous logic in this
method will fail.
[1]https://review.opendev.org/c/openstack/neutron/+/907402
Partial-Bug: #2052821
Change-Id: Ieac603b0c7a940693a12170b453eea39dd0eaa47