uid_number and gid_number should only be necessary inside a storage driver

[butonic]

Currently, the reva user has a property for uid_number and gid_number. They are used to chown files and set acls in drivers like eos. But this is a driver and even os specific property that may even change over time.

Drivers that integrate with the os should look up the current numeric ids based on the current username. They should not use the cs3 userprovider to look it up, as it is an implementation detail.

• eos can use os/user.Lookup() and should integrate with the os sssd anyway, @labkode right?
• cephs same
• and othet drivler that wants to set ACLs as well

Or am I missing something? I know it was me that added them as opaque ids ... a long time ago. See owncloud/ocis#444 (comment) where I was digging into related problems.

cc @rhafer