utils/types: toggle_set: add "~~" semantics

Add support for "~~" special value that indicates that, when merging
into another set, all values from that set should in fact be dropped.
Apart from the unique merge semantics, "~~" just like any other "~"
value.

wa/framework/configuration/core.py

@@ -957,6 +957,8 @@ class JobGenerator(object):
 
     def disable_augmentations(self, augmentations):
         for entry in augmentations:
+            if entry == '~~':
+                continue
             if entry.startswith('~'):
                 entry = entry[1:]
             try:

wa/framework/configuration/parsers.py

@@ -306,7 +306,10 @@ def _construct_valid_entry(raw, seen_ids, prefix, jobs_config):
             workload_entry[name] = value
 
     if "augmentations" in workload_entry:
-        jobs_config.update_augmentations(workload_entry["augmentations"])
+        if '~~' in workload_entry['augmentations']:
+            msg = '"~~" can only be specfied in top-level config, and not for individual workloads/sections'
+            raise ConfigError(msg)
+        jobs_config.update_augmentations(workload_entry['augmentations'])
 
     # error if there are unknown workload_entry
     if raw: