build/install: Fix missing arch list from Release

build/install: Fix missing arch list from Release

During the previous attempt to fix the local mirror rebuild by
reworking `atp-ftparchive` configuration, we removed the arch
list from the main configuration, which led to the local Release
file missing "Architectures" entries.

This led to "/etc/fuel_openstack_arch" not being populated,
binfmt not starting on the first boot, and bootstrap build failing.

Bring back the "Architecture" config line to restore the old
behavior.

JIRA: ARMBAND-163

Change-Id: Ica64fcd60ed4e0185d2cab5457603b42d3d55af8
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>