Allow re-running of Firstboot phase or custom actors in Leapp after upgrade has completed

Issue

• During Leapp upgrade, custom actors are executed during the Firstboot phase to make customer specific changes to the RHEL 8 deployment.
• A custom actor may fail to execute unexpectedly.
• The upgrade to RHEL 8 succeeds even though some of the custom actors may have failed.
• It is not possible to re-run the Firstboot phase.