Define NFS mount options for shared folders

[laoneo]

On Ubuntu 16.04, the following error happens when using flysystem:

file_put_contents(): Exclusive locks are not supported for this stream

This can be fixed with the nolock mount option as suggested in the issue here thephpleague/flysystem#445.

[stevenrombauts]

@laoneo Thank you for this fix. This should work on all systems if I understand correctly but ideally we make it possible to override the mount configuration options. I've created a new ticket for that (#77).