ulri currently takes the list of bots defined in $config->{bot} to find
the list of architecture for which it should build packages.

With this change, it is now possible to define in $config->{$arch} the
list of architectures for each distro. If it is not defined for a
distro, $config->{$arch}->{default} is used.

emi is using $config->{mandatory_arch} to find the list of mandatory
architectures with successful build required to upload a package.
With this change it is now also possible to set per distro
mandatory_arch in $config->{mandatory_arch}->{$distro}. If no mandatory
architecture is defined for a distro, then $config->{mandatory_arch}->{default}
is used.