Attempt to preserve screen trace

This adds a new YAML config option to save the screen trace after a SIPp run.

It works except that the runner has to run SIPp via sudo, which means that it has the PID of sudo, and not of SIPp. This causes the PID in the filename to be wrong (line 83)

Todo:
- [x] Fix SIPP pid to locate the correct screen file
- [x] Specs
- [x] Documentation