Code review comment for lp:~smoser/maas-images/lp1626810

Note... this will create an empty products tree.
I'm not entirely certain that all other tools will handle that
empty products tree. Other tools may need to be updated to address this.

Example:

$ ./bin/meph2-build powerpc yakkety 20101001 /tmp/foo.img out.d
arch 'powerpc' is not supported for release 'yakkety'.
Release has architectures: {'ppc64el', 'arm64', 's390x', 'i386', 'amd64', 'armhf'}.
To support, add kernel info to config.

$ for f in $(find out.d -type f); do echo == $f ==; cat $f; done
== out.d/streams/v1/com.ubuntu.maas:daily:v2:download.json ==
{
 "content_id": "com.ubuntu.maas:daily:v2:download",
 "datatype": "image-downloads",
 "format": "products:1.0",
 "products": {},
 "updated": "Fri, 23 Sep 2016 16:14:25 +0000"
}
== out.d/streams/v1/index.json ==
{
 "format": "index:1.0",
 "index": {
  "com.ubuntu.maas:daily:v2:download": {
   "datatype": "image-downloads",
   "format": "products:1.0",
   "path": "streams/v1/com.ubuntu.maas:daily:v2:download.json",
   "products": [],
   "updated": "Fri, 23 Sep 2016 16:14:25 +0000"
  }
 },
 "updated": "Fri, 23 Sep 2016 16:14:25 +0000"
}