Fix issue with plot_rapthor_timing script

After Rapthor learned the new initial_image command, the script to plot timings broke. Reason is that the initial_image command is executed only once, and doesn't have a trailing _<cycle-nr> in the directory name. The solution for now is to add a trailing _1 to those operations (currently only initial_image) that do not have one.

Closes RAP-868