config: rename external-snapshots to snapshot-as-volume-chain

Not perfect but now it's still easy to rename and the new variant fits
a bit better to the actual design and implementation.

Add best-effort migration for storage.cfg, this has been never
publicly released after all.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/PVE/Storage/NFSPlugin.pm

@@ -104,7 +104,7 @@ sub options {
         'create-subdirs' => { optional => 1 },
         bwlimit => { optional => 1 },
         preallocation => { optional => 1 },
-        'external-snapshots' => { optional => 1, fixed => 1 },
+        'snapshot-as-volume-chain' => { optional => 1, fixed => 1 },
     };
 }