Update exposure_pipeline.py to include refpix

[stscijgbot-rstdms]

Issue RCAL-647 was created on JIRA by David Davis:

Initially the refpix step was left out of the pipeline due to external restrictions. Those have now been relaxed so add the refpix step to the standard processing.

 

The refpix_step needs to set the cal_log step to "COMPLETE" when done and set the suffix to "refpix" instead of using the fallback suffix of "refpixstep".