Add the option to pass a custom path for the multilib config files

The default is /usr/share/pungi/multilib/, pass --multilibconf to
override this.

This also adds multilib.init() so that an import of multilib doesn't
immediately setup the classes.

(cherry picked from commit 234524296fd53871aed64690cf6a7d5849ca154a)

pungi/gather.py

@@ -106,6 +106,7 @@ class PungiBase(object):
 
     def __init__(self, config):
         self.config = config
+        multilib.init(self.config.get('pungi', 'multilibconf'))
 
         # ARCH setup
         self.tree_arch = self.config.get('pungi', 'arch')