Make ValidationError consistent with rest-framework

This is silly, but rest-framework returns an error with a '.' at the
end.  Test automation validates the error responses. Rather than special
case this condition, I've chosen to raise an Exception with a msg
consistent with rest-framework.

awx/api/serializers.py

@@ -1308,7 +1308,7 @@ class JobOptionsSerializer(BaseSerializer):
     def validate_project(self, attrs, source):
         project = attrs.get('project', None)
         if not project and attrs.get('job_type') != PERM_INVENTORY_SCAN:
-            raise serializers.ValidationError("This field is required")
+            raise serializers.ValidationError("This field is required.")
         return attrs
 
     def validate_playbook(self, attrs, source):