Add cluster-name to external-openstack-cloud-controller-manager (#7055)

If cluster-name is not set, the default value "kubernetes" is used.
The loadbalancees created by Kubernetes follow the format:
  kube_service_clusterName_serviceNamespace_serviceName
If 2 clusters create a loadbalancer for the same service in the same
namespace, they will share the same non-working loadbalancer.

Signed-off-by: Cedric Hnyda <cedric.hnyda@itera.io>