There is now a single option `multilib`, that maps variants and arches
to multilib methods. This replaces old `multilib_methods` option.
Multilib arches are implicitly deduced instead of using the
`multilib_arches` option.
The test compose is updated to only enable multilib on Server and its
addons.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>