Toggle multilib per variant

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>

pungi/util.py

@@ -215,6 +215,11 @@ def get_arch_variant_data(conf, var_name, arch, variant):
     return result
 
 
+def is_arch_multilib(conf, arch):
+    """Check if at least one variant has multilib enabled on this variant."""
+    return bool(get_arch_variant_data(conf, 'multilib', arch, None))
+
+
 def resolve_git_url(url):
     """Given a url to a Git repo specifying HEAD as a ref, replace that
     specifier with actual SHA1 of the commit.