allow some missing path sections in runroot config

Fixes: https://pagure.io/koji/issue/527

Before path sections were expected to have zero-based numbering. If some
item was missing, parsing ended there.

Now we are more benevolent and we pick all path\d+ sections and sort
them by ordering number.

tests/test_plugins/test_runroot_builder.py

@@ -31,6 +31,9 @@ class FakeConfigParser(object):
     def read(self, path):
         return
 
+    def sections(self):
+        return self.CONFIG.keys()
+
     def has_option(self, section, key):
         return section in self.CONFIG and key in self.CONFIG[section]