Fix validation of campaign configs

This merge switches to first merging user and campaign config
files and only validating after the merge.  This slightly
complicates the logic for getting the campaign (if any) because
defaults have not yet been parsed at that point.  But it does
mean we only validate once (so only one validation success is
printed) and it means that defaults will not override the
campaign config options, as was happening before this merge.

zppy/__main__.py

@@ -31,21 +31,23 @@ def main():
     # Read configuration file and validate it
     default_config = os.path.join(templateDir, "default.ini")
     user_config = ConfigObj(args.config, configspec=default_config)
-    _validate_config(user_config)
-    campaign = user_config["default"]["campaign"]
+    if "campaign" in user_config["default"]:
+        campaign = user_config["default"]["campaign"]
+    else:
+        campaign = "none"
     if campaign != "none":
         campaign_file = os.path.join(templateDir, "{}.cfg".format(campaign))
         if not os.path.exists(campaign_file):
             raise ValueError(
                 "{} does not appear to be a known campaign".format(campaign)
             )
-        config = ConfigObj(campaign_file, configspec=default_config)
-        _validate_config(config)
-        # merge such that user_config takes priority
-        config.merge(user_config)
+        campaign_config = ConfigObj(campaign_file, configspec=default_config)
+        # merge such that user_config takes priority over campaign_config
+        campaign_config.merge(user_config)
+        config = campaign_config
     else:
-        # no need to merge
         config = user_config
+    _validate_config(config)
 
     # Add templateDir to config
     config["default"]["templateDir"] = templateDir