There already were config options tree_arches and tree_variants, but the
filtering was done at the compose level and there was not interaction
between the two options. This led to problems when a variant would have
all its arches filtered out.
This patch moves the filtering to the variant loader. It adds better
logging, so whenever a variant is filtered (for any reason), it will be
explicitly stated in the logs.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
