Add custom file permissions for cStor ala the Dynamic NFS Provisioner

[dborman-hpe]

We are having a push to run as many containers as possible as non-root. I was converting a non-root pod from a CephFS PVC to an OpenEBS/cStor PVC, but the root of the PVC is root:root 0755, which means the non-root container cannot create files.

I was able to get this working by adding:
securityContext:
fsGroup: 65534
runAsUser: 65534
to the pod specification. With this the container is still running as non-root, but kubernetes changes the group owner/permissions in the PVC to the specified group, and then the container can create files.

It would be nice to be able to set the ownership and/or permissions on the root directory of the PVC when it is created.