server: add option to dynamically create devices

add module which scans a connection and registers new devices depending
on the answer.
* change module initialization to demand-based
* move code from server to dispatcher
- remove intermediate step in Attached __get__

TODO:
  factor out dispatcher (regards to playground)
  discuss factoring out of module creation code from server AND
  dispatcher

Change-Id: I7af959b99a84c291c526aac067a4e2bf3cd741d4
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/31470
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

cfg/test_cfg.py

@@ -10,6 +10,22 @@ The needed fields are Equipment id (1st argument), description (this)
      'tcp://10768',
 )
 
+Mod('attachtest',
+    'frappy_demo.test.WithAtt',
+    'test attached',
+    att = 'LN2',
+)
+
+Mod('pinata',
+    'frappy_demo.test.Pin',
+    'scan test',
+)
+
+Mod('recursive',
+    'frappy_demo.test.RecPin',
+    'scan test',
+)
+
 Mod('LN2',
     'frappy_demo.test.LN2',
     'random value between 0..100%',