Make the Farm retrieve APIs return the name of the Farm's owner

Field 'owner' was added to the Farm model in commit 2dd4338.
The owner's name should be returned through the Farm retrieve APIs,
instead of the owner's primary key. This was overlooked on a previous pull request.
(#582)

Author: hoangpn

promgen/serializers.py

@@ -106,6 +106,7 @@ def to_representation(self, obj):
 
 
 class FarmSerializer(serializers.ModelSerializer):
+    owner = serializers.ReadOnlyField(source="owner.username")
     url = WebLinkField()
 
     class Meta:

promgen/tests/examples/rest.farm.1.json

@@ -3,7 +3,7 @@
   "url": "http://promgen.example.com/farm/1",
   "name": "test-farm",
   "source": "promgen",
-  "owner": null,
+  "owner": "admin",
   "hosts": [
     {
       "name": "host.example.com",

promgen/tests/examples/rest.farm.json

@@ -4,6 +4,6 @@
     "url": "http://promgen.example.com/farm/1",
     "name": "test-farm",
     "source":"promgen",
-    "owner": null
+    "owner": "admin"
   }
 ]