Switch taskflow to use the unittests slave script

Instead of using the tox slave script which doesn't report back
very meaningful (if any) testr results to review system use the
unittests one which does provide this information just like the
standard jobs.

Closes-Bug: 1273850

Change-Id: I11ff92ed719f665064336a1fcf8111d1d652e991
Merge "Switch taskflow to use the unittests slave script"