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>

tests/data/dummy-pungi.conf

@@ -42,8 +42,11 @@ greedy_method = "build"
 check_deps = False
 hashed_directories = True
 
-multilib_arches = ["ppc64", "x86_64", "s390x"]
-multilib_methods = ["devel", "runtime"]
+multilib = [
+    ('^Server.*$', {
+        '*': ['devel', 'runtime']
+    }),
+]
 
 additional_packages = [
     ('^Server$', {