Enhancement: provide options for rolling back an unsuccessful switch attempt

[Djelibeybi]

The previous restore_repos() function was well-intentioned, but wasn't updated to handle the new release RPMs, so it turned into more harm than good. It has now been removed.

However, it would be good if we always grabbed a list of RPMs installed before the script makes any changes so that in the event of a problem, we could at least tell the user what new RPMs were installed (so that they could be manually removed).

We should also take a backup of all repo definitions prior to starting so that they can be provided at this point.

[mark-au]

For context, the function was removed in 786e581