Replace zfsexists binary with Python script

The zfsexists binary we have compiled doesn't work on Ubuntu 22.04 because the underlying ZFS library version has increased. I could recompile it, but then I'd need a different compiled version for Focal. These days there is a Python module for talking to ZFS which is available on both Focal and Jammy with an identical interface. This change replaces the binary zfsexists with a Python script.