Default RTEs implementation in Python backends
New RTEs enable/disable rewrite also adds new feature to the shell-based backends: default RTEs. Default RTEs are RTEs that used for every job regardless of job description. Technically those RTEs are added to the end of RTEs list (after RTEs that comes from job description) and than operates as always.
Python backends now missing this addition. I had not implemented it myself in Python and think @skou can do it more correctly.
Relevant shell code: d2188a88