Make default directory permissions configurable

[aRestless]

https://github.com/kubernetes-sigs/nfs-subdir-external-provisioner/blob/master/cmd/nfs-subdir-external-provisioner/provisioner.go#L112

The directory permissions on created volumes are hardcoded to 777.

Some applications (Nextcloud in my case) will fail if their data directories have too open permissions, and rightfully so. I would suggest an environment variable that can override the default directory permissions to, at least, 770.

I'll gladly open up a PR, but wanted to check in first if there would be downsides to this change that I'm unaware of.

[yonatankahana]

if there would be downsides to this change that I'm unaware of.

test it yourself :)
if it will work for you, it would probably work for many and PR will be appreciated.

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