Add Patternfly styling for progress-bar

Bootstrap has a progress bar but Patternfly hasn't styled it in the
flatter style.  The colors come from here:

https://www.patternfly.org/visual-styles/

We're using the Patternfly and progress bar in the Cockpit project, and are styling this to match with Patternfly ourselves, so we thought we'd contribute the fix upstream. See:

https://github.com/cockpit-project/cockpit/pull/309