config: add 'include' and 'override'

including a config file and overriding some properties is
helpful when we do not want to modify the original config
but run it with sligthly different properties.

this involves some redesign a.o.:
- modules are collected in a dict instead of a list in
  order for 'override' to find the related module
- checking for duplicates happens in the Collector

Do not warn when included file does not end with '_cfg.py',
as this may be intentional, in case a file is only used
via 'include' and not as cfg file alone.

+ remove unused method Collector.append
+ complain with specific error message when Node is not given

Change-Id: Id568f04d6d84622ef2547412eb6f288fcebf986f
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/36357
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_config.py

@@ -92,8 +92,8 @@ def test_cfg_not_existing(direc, log):
 def collector_helper(node, mods):
     n = NodeCollector()
     n.add(*node)
-    m = Collector(Mod)
-    m.list = [Mod(module, '', '') for module in mods]
+    m = Collector()
+    m.modules = {module: Mod(module, '', '') for module in mods}
     return n, m