Curtin Userdata customization: avoid creation of swap file in MAAS

*** /etc/maas/preseeds/curtin_userdata_ubuntu_amd64_generic_xenial~     2017-10-04 13:14:20.043937203 +0000
--- /etc/maas/preseeds/curtin_userdata_ubuntu_amd64_generic_xenial      2017-03-06 15:23:15.838938002 +0000
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*** 13,22 ****
    driver_03_update_install: ["sh", "-c", "apt-get update --quiet && apt-get --assume-yes install {{driver['package']}}"]
    driver_04_load: ["sh", "-c", "depmod && modprobe {{driver['module']}}"]
  {{endif}}
+ swap:
+   filename: swap.img
+   size: 0B
+   maxsize: 0B
  late_commands: