slap/slap: Unwrap json-in-xml

This makes it so that both `xml` and `json-in-xml` encoding for
instance parameters are accepted by all SRs, regardless of whether
they use `slapos.cookbook:slapconfiguration` in `.serialised` mode.

slapos/slap/slap.py

@@ -548,9 +548,15 @@ class ComputerPartition(SlapRequester):
       setattr(software_instance, '_%s' % key, value)
 
     if raw_information["data"].get("text_content", None) is not None:
-      setattr(software_instance, '_parameter_dict', xml2dict(raw_information["data"]['text_content']))
+      parameter_dict = xml2dict(raw_information["data"]['text_content'])
+      if len(parameter_dict) == 1:
+        try:
+          parameter_dict = json.loads(parameter_dict['_'])
+        except (KeyError, ValueError, TypeError):
+          pass
     else:
-      setattr(software_instance, '_parameter_dict', {})
+      parameter_dict = {}
+    setattr(software_instance, '_parameter_dict', parameter_dict)
 
     setattr(software_instance, '_requested_state', raw_information["data"]['slap_state'])
     setattr(software_instance, '_connection_dict', raw_information["data"]['connection_parameter_list'])