Identify duplicated code and merge them

[xlc]

          I found there are too many duplicated code for a new system parachain. This is a code smell and should be addressed at some stage. We will have more system parachains and we need to reduce the overhead of making one before making more system parachains.

Originally posted by @xlc in #319 (review)

[joepetrowski]

There isn't very much overhead, can set up a new one in a morning. Feel free to make a PR but we (Parity) have already had a lot of people restructure code to have less duplication where it makes sense.