do not override the sourcedeb container codename

This causes our "saucy" sourcdebs to be built with trusty content. 

For example the Project IrelUbu__genmsg__ubuntu_saucy__source
creates: 0.5.5-0trusty

It is true that the sourcedeb container image could be standardized on [trusty](https://github.com/ros-infrastructure/ros_buildfarm/blob/master/templates/release/sourcedeb_task.Dockerfile.em#L1). But the os_code_name is used in 2 other places which set the apt sources and get_source.py arguments which leads to cross talk. 
https://github.com/ros-infrastructure/ros_buildfarm/blob/master/templates/release/sourcedeb_task.Dockerfile.em#L16
https://github.com/ros-infrastructure/ros_buildfarm/blob/master/templates/release/sourcedeb_task.Dockerfile.em#L49

I tested this works on my test deployment. It created: 0.5.5-0saucy