feat: Make Kube-OVN namespace configurable with default value (#5069)

Allow customization of the Kube-OVN namespace by introducing a default value of 'kube-system' using parameter expansion

Signed-off-by: Mengxin Liu <liumengxinfly@gmail.com>

Author: oilbeater

dist/images/kubectl-ko

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -euo pipefail
 
-KUBE_OVN_NS=kube-system
+KUBE_OVN_NS=${KUBE_OVN_NS:-kube-system}
 WITHOUT_KUBE_PROXY=${WITHOUT_KUBE_PROXY:-false}
 OVN_NB_POD=
 OVN_SB_POD=