This artifact is no longer used in the "Logical_Router" registers (in
the "options" field) to mark this "Logical_Router" as unhosted. A
"Logical_Router" is considered as unhosted if the gateway
"Logical_Router_Ports" have no "chassis" set.
This artifact is also used to create a "Gateway_Chassis" register
pointing to a inexisting invalid chassis called
"neutron-ovn-invalid-chassis". Any "Logical_Router_Port" not bound
to a chassis will have no value in "gateway_chassis" (NOTE1).
NOTE1: this is valid now with the current two OVN L3 schedulers that
use "gateway_chassis" to schedule the "Logical_Router_Port" of a
router. In a future, we can consider using "ha_chassis_group" for
scheduling.
Partial-Bug: #2052821
Related-Bug: #2019217
Change-Id: I12717936fe2bc188545309bacb8a260981f14c88
fa3223bb9d