Bump the version of Debian net-installer to 9.4.0 (#2485)

Author: khos2ow

tools/appliance/systemvmtemplate/template.json

@@ -38,8 +38,8 @@
       "disk_interface": "virtio",
       "net_device": "virtio-net",
 
-      "iso_url": "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-9.3.0-amd64-netinst.iso",
-      "iso_checksum": "8775231d6f56a3d8f116eb64fe048f5cbd2ea0f8c092a1cb7608bcb4106f9c85cb69ce68f53bd381019ab40f1c0316843036daf3fd9107c81c58a240334cc747",
+      "iso_url": "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso",
+      "iso_checksum": "345c4e674dc10476e8c4f1571fbcdba4ce9788aa5584c5e2590ab3e89e7bb9acb370536f41a3ac740eb92b6aebe3cb8eb9734874dd1658c68875981b8351bc38",
       "iso_checksum_type": "sha512",
 
       "vm_name": "systemvmtemplate",