bundles/zfs: add option to disable snapshots for dataset tree

bundles/zfs/files/zfs-auto-snapshot

@@ -31,8 +31,15 @@ label = argv[1]
 with open('/etc/zfs-snapshot-config.json', 'r') as fp:
     metadata = loads(fp.read())
 
-output = check_output(['zfs', 'list', '-H', '-o', 'name']).decode('UTF-8')
-datasets = set(output.splitlines())
+datasets = set()
+for line in check_output(['zfs', 'list', '-H', '-o', 'name']).splitlines():
+    line = line.decode('UTF-8')
+
+    for prefix in metadata.get('snapshot_never', set()):
+        if dataset.startswith(prefix):
+            break
+    else:
+        datasets.add(line)
 
 default_retain = metadata['retain_defaults'][label]
 now = datetime.now().strftime('%F-%H%M')