[autotest] Fix awkward Django usage

Django model fields are usually accessed by attribute.
get_object_dict is like using __dict__ to get a dict representation.

BUG=None
TEST=None

Change-Id: I02a18364a525ef934abca715f6b0b8f22363270a
Reviewed-on: https://chromium-review.googlesource.com/437822
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/frontend/afe/rpc_utils.py b/frontend/afe/rpc_utils.py
index cd79b88..b98ecc0 100644
--- a/frontend/afe/rpc_utils.py
+++ b/frontend/afe/rpc_utils.py
@@ -1400,4 +1400,4 @@
     if not hosts:
         return None
     else:
-        return hosts[0].get_object_dict()['hostname']
+        return hosts[0].hostname