Gateways information are not exposed via OpenStack cli

Using OpenStack/OVN, routers gateways information are only available via Open Virtual Network northbound db management utility (ovn-nbctl) [0]. This tool requires root access to the OVN NB DB leader.

As an operator, I would like to use OpenStack API to retrieve similar information to what is exposed by ovn-nbctl and being able to perform set/del of gateways routers.

Current features missing:

1. get list of ovn-chassis assigned to a gateway with priority.

   This is currently available via:

   ovn-nbctl lrp-get-gateway-chassis <lrp-port>

   but should be available via something similar to:

   openstack network agent list --router <uuid>

2. Set gateway chassis for port with a priority

   This is currently available via:

   ovn-nbctl lrp-set-gateway-chassis <lrp-port> <chassis> [priority]

   but should be available via something similar to:

   openstack network agent set --router <uuid> [--priority <priority>] <agent-uuid>

3. Deletes gateway chassis from port

   This is currently available via:

   ovn-nbctl lrp-del-gateway-chassis <lrp-port> <chassis>

   but should be available via something similar to:

   openstack network agent unset --router <uuid> <agent-uuid>

[0] https://manpages.ubuntu.com/manpages/bionic/man8/ovn-nbctl.8.html