Add tests for OVN chassis cluster member role

[tomponline]

https://github.com/lxc/lxd/pull/10119 introduce the concept of an OVN chassis cluster member role, but did not have any tests to accompany it.

We need to introduce tests for that feature to the https://github.com/lxc/lxc-ci/blob/master/bin/test-lxd-network-ovn test suite (or perhaps a separate file to setup a cluster).