samba-tool: give cache_loader pseudo-dict a .get() method

This makes it more dict-like, and makes the next patch (adding
samba-tool help) simpler.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

python/samba/netcmd/main.py

@@ -36,6 +36,12 @@ class cache_loader(dict):
                                  'cmd_%s' % attr)()
         return dict.__getitem__(self, attr)
 
+    def get(self, attr, default=None):
+        try:
+            return self[attr]
+        except KeyError:
+            return default
+
     def iteritems(self):
         for key in self:
             yield (key, self[key])