systemvmtemplate: Fix Debian 9 iso url (apache#3582)

Since version 10.0.0 of Debian has become stable, the URL of the Debian 9.9.0 ISO files has changed from current to archive.

The old URL returns a 404 and crash the build of systemvm templates.

Author: dpassante

tools/appliance/systemvmtemplate/template.json

@@ -38,7 +38,7 @@
       "disk_interface": "virtio",
       "net_device": "virtio-net",
 
-      "iso_url": "https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-9.9.0-amd64-netinst.iso",
+      "iso_url": "https://cdimage.debian.org/cdimage/archive/9.9.0/amd64/iso-cd/debian-9.9.0-amd64-netinst.iso",
       "iso_checksum": "42d9818abc4a08681dc0638f07e7aeb35d0c44646ab1e5b05a31a71d76c99da52b6192db9a3e852171ac78c2ba6b110b337c0b562c7be3d32e86a105023a6a0c",
       "iso_checksum_type": "sha512",