Bump ZFS usage to 50% RAM

It's a lot, but should be dealable on most machines

ansible/roles/zfs/tasks/main.yml

@@ -1,6 +1,6 @@
 - name: Calculate ZFS ARC size
   set_fact:
-    zfs_arc_size: "{{ (ansible_memtotal_mb * 1024 * 1024) * 0.2 }}"
+    zfs_arc_size: "{{ (ansible_memtotal_mb * 1024 * 1024) * 0.5 }}"
 
 - name: Enable zfs module
   modprobe: