Our 'cover' tox job does not work with release of 'coverage' package v4.0
It fails on option 'ignore-errors' in config section '[report]'.
So, replace usage of this option with CLI option '--ignore-errors'.
Change-Id: I404e3cb63453b0245ac05b4d3cb5780e5b8b7f9a
Closes-Bug: #1497897