Clarify that user can not user `/stash` as `mountPath` for local backend

[hossainemruz]

Reason:
We have stash binary in root directory. Hence, if the user try to use /stash or /stash/* as mountPath, he will get the following error:

Message:      OCI runtime create failed: container_linux.go:345: starting container process caused "process_linux.go:430: container init caused \"rootfs_linux.go:58: mounting \\\"/var/lib/kubelet/pods/925c4a63-fa1d-11e9-b97a-0050560cc8c2/volume-subpaths/stash-local/stash/4\\\" to rootfs \\\"/var/lib/docker/overlay2/6cd0d369135dd0e45bdd29789ee028012dbab7b912204e135cbfd00b4c51fe7e/merged\\\" at \\\"/var/lib/docker/overlay2/6cd0d369135dd0e45bdd29789ee028012dbab7b912204e135cbfd00b4c51fe7e/merged/stash\\\" caused \\\"not a directory\\\"\"": unknown: Are you trying to mount a directory onto a file (or vice-versa)? Check if the specified host path exists and is the expected type

[tlvenn]

Could an admission hook reject the repository or the blueprint if such mountPath is being used ?

[hossainemruz]

Good idea.