You can use the configmap to store your certificate and mount it to your deployment.
Configmap will get auto-updated inside the Running POD without restarting the POD. So you have to just mount the config map once and changes will be autp updated to all available replicas of deployment without any restart and disruption.
Read my article : Update configmap without restarting POD
Store you cert in configmap and mount it to POD
: People consider configmap as insecure option to store secure data, if someone has your cluster access they can watch or view your certificates. If that's not issue in your case it is made for storing configuration only so will work like charm.