dev-scripts(setup-rancher-clusters): Fix public_hostname without IP address

[p-se]

Fixes dev/setup-rancher-clusters where public_hostname may end up
being without IP address.

E.g.

+ upstream_ctx=k3d-upstream
+ downstream_ctx=k3d-downstream1
+ rancherpassword=rancherpassword
+ public_hostname=
+ '[' -z '' ']'
+ kubectl get service -n kube-system traefik -o 'jsonpath={.status.loadBalancer.ingress[0].ip}'
Error from server (NotFound): services "traefik" not found
+ sleep 3
+ kubectl get service -n kube-system traefik -o 'jsonpath={.status.loadBalancer.ingress[0].ip}'
Error from server (NotFound): services "traefik" not found
+ sleep 3
+ kubectl get service -n kube-system traefik -o 'jsonpath={.status.loadBalancer.ingress[0].ip}'
Error from server (NotFound): services "traefik" not found
+ sleep 3
+ kubectl get service -n kube-system traefik -o 'jsonpath={.status.loadBalancer.ingress[0].ip}'
++ kubectl get service -n kube-system traefik -o 'jsonpath={.status.loadBalancer.ingress[0].ip}'
+ ip=
+ public_hostname=.sslip.io

Also try again if getting a login token failed for registering a downstream cluster as shown below:

+ public_hostname=172.18.0.2.sslip.io
+ ./.github/scripts/register-downstream-clusters.sh
+ public_hostname=172.18.0.2.sslip.io
+ cluster_downstream=k3d-downstream1
++ kubectl config current-context
+ ctx=k3d-upstream
+ command -v rancher-cli
+ command -v rancher
+ rancher_cli=rancher
++ kubectl get secret --namespace cattle-system bootstrap-secret -o 'go-template={{.data.bootstrapPassword|base64decode}}'
+ rancherpassword=rancherpassword
++ curl -k -s -X POST 'https://172.18.0.2.sslip.io/v3-public/localProviders/local?action=login' -H 'Content-Type: application/json' -d '{"username":"admin","password":"ranc
herpassword"}'
+ login_response='{"baseType":"error","code":"ServerError","message":"Server error while authenticating","status":500,"type":"error"}'
++ echo '{"baseType":"error","code":"ServerError","message":"Server error while authenticating","status":500,"type":"error"}'
++ jq -r .token
+ token=null
+ '[' null = null ']'
+ echo 'Failed to get authentication token'
Failed to get authentication token
+ exit 1

[weyfonk]

nit: is there a specific reason for a sleep 5 where the previous until...do construct used a sleep 3?