Revisit the unmanaged vfolder with the latest storage-proxy architecture

[achimnol]

Unmanaged vfolders allow superadmins to map a random host path to a vfolder, in case that users need to cooperate with external systems which use a specific host path or sub-path in shared network volumes, not following our quota-scope and vfolder path mangling scheme.

For instance, a vfolder created with ./backend.ai vfolder create --unmanaged unfolder1 /home/joongi/unfolder1,

Issue: unmanaged vfolders do not work with the current backend-aware storage-proxy implementation with explicit vfolder host configurations, as the vfolder-host information is missing for them.

Let's update the implementation as follows:

• If the host path of an unmanaged vfolder contains a known vfolder host's mount path, let's follow the configuration of the matched vfolder host.
• Otherwise, let's introduce a concept of "ghost vfolder host", which provides a default set of vfolder-host permission configurations.