rpm: Rename _base variables to _main
rpm: Rename _base variables to _main

This removes some awkwardness.

While at it, better document the expectations in terms of names
and contents for the variables used for package lists.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

kubevirt/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt