k3s plugin - add PV option

[taraszka]

Description of feature

Old legacy https://github.com/dokku/dokku-scheduler-kubernetes can add PV for an App build by dokku and deployed onto Kubernetes.
The way how it works is simple:

1. it claims PV using this template: https://github.com/dokku/dokku-scheduler-kubernetes/blob/master/templates/pvc.json.sigil
2. it mounts volume here https://github.com/dokku/dokku-scheduler-kubernetes/blob/master/internal-functions#L776

dokku params are also quite clear:

# dokku scheduler-kubernetes:add-pvc ${pvc_name} ${pvc_size} --access-mode ReadWriteMany --namespace ${namespace} --storage-class-name ${storage_class}
# dokku scheduler-kubernetes:mount ${app_name} ${pvc_name} ${path}'

Note: dokku-scheduler-kubernetes, for PVC size, only accepts integer value that represents Mi. So for 2 Gi Volume, you have to say: 2000. Would be nice if k3s would accept Mi, Gi, Ti, so param would be: (..):add-pvc my-awsome-pvc 2Gi --access-mode (...)