Add --repo-post option to bkr workflow commands

New option adds the given repo URL using ks_appends so it is
available to tasks, but not during the initial installation
of packages.

Includes tests for both the --repo-post and --repo options.

Based on an initial patch by Gurhan Ozen

Bug: 867087
Change-Id: I216f1450e5a380ecdc2098dfb0a309ce85a0d479