Arch-specific code in generic preseed

[Gavin Panella]

contrib/preseeds_v2/generic contains:

{{if node.architecture in {'i386', 'amd64'} }}
d-i     mirror/http/hostname string archive.ubuntu.com
d-i     mirror/http/directory string /ubuntu
{{else}}
d-i     mirror/http/hostname string ports.ubuntu.com
d-i     mirror/http/directory string /ubuntu-ports
{{endif}}

This should go instead into contrib/preseeds_v2/i386_generic and
.../amd64_generic (or .../x86_generic, symlinked from the two
aforementioned locations).

The reason is so that arch-specific settings can be dropped in by
3rd party packages, or by site admins, by simply placing a
properly named file into a .d directory. Doing arch detection
like this interferes with that.

[Robie Basak (racb)]

When I wrote this, I tried and failed to use arch-specific templates instead. The template mechanism seemed inverted in a way that made this impossible (or at least I failed to find a way).