fix: datastore: make relative_group_path() return relative path

fix: datastore: make relative_group_path() return relative path

previously the BackGroup trait used the datastore's
namespace_path() method to construct a base path. this would result in
it returning an absolute path equivalent to full_group_path(). use
the namspace's path() method instead to get a relative path, in-line
with backup_dir's relative_path().

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>