etcd cannot be clustered with itself via CMR

[Jeff Hillman]

juju 2.9.18
k8s 1.22
openstack ussuri
ubuntu focal

In the event of attempting to stretch the kubernetes control plane across multiple openstack regions, etcd cannot relate to itself via the cluster endpoint.

Steps to recreate:

```
On RegionOne

$ juju add-model r1

### deploy etcd and offer relevant endpoints for CMR

$ juju deploy cs:~containers/etcd --constraints instance-type=4-4-16
$ juju offer etcd:cluster etcd-cluster
$ juju offer etcd:db etcd-db

$ juju add-model r2

### deploy etcd master and attempt to consume offers from r1

$ juju deploy cs:~containers/etcd --constraints instance-type=4-4-16

### attempt etcd to consume offers

$ juju add-relation etcd r1:admin/r1.etcd-cluster
ERROR no relations found

```

Easyrsa can be offered/consume just fine with the same scenario.

Also, the etcd:db endpoint can be succesfully offered via CMR to the kubernetes-master charm

This inhibits the ability to span the control plane across openstack regions/cloud for maximum high-availability