auto-t: only call set_value for changed values in default()

The default() method was added for convenience but was extending the
test times significantly when the hostapd config was lengthy. This
was because it called set_value for every value regardless if it
had changed. Instead store the current configuration and in default()
only reset values that differ.

autotests/util/hostapd.py

@@ -75,6 +75,8 @@ class HostapdCLI(object):
         if self._default_config.get('vendor_elements', None) == None:
             self._default_config['vendor_elements'] = ''
 
+        self._current_config = self._default_config.copy()
+
         if not self.interface:
             raise Exception('config %s not found' % config)
 
@@ -170,6 +172,8 @@ class HostapdCLI(object):
         if self._default_config.get(key, None) == None:
             raise Exception("Untracked setting '%s'! Please set default in hostapd config" % key)
 
+        self._current_config[key] = value
+
         cmd = self.cmdline + ['set', key, value]
         ctx.start_process(cmd).wait()
 
@@ -200,7 +204,10 @@ class HostapdCLI(object):
 
     def default(self):
         for k, v in self._default_config.items():
-            self.set_value(k, v)
+            # Only bother setting the value if it differs
+            if self._current_config[k] != v:
+                self.set_value(k, v)
+                self._current_config[k] = v
 
         self.reload()