ceph-perf: install mdutils and run CBT under gh-venv
cbt imports mdutils but ceph/cbt master requirements.txt does not
list it. Install mdutils into gh-venv, put the venv on PATH for
run-cbt.sh, and invoke compare.py via python3 so its /usr/bin/python3
shebang is not used. Only skip CBT runs when the archive dir already
has results, and fail the step if the archive is empty after run-cbt.