With self-contained configuration, the repository definitions
will be predictable based on the target architecture and avoid
leaking anything from the host.
This change makes it so that the ARM image creation script uses DNF
instead of urpmi. This affords several advantages:
1. Mageia images can be created from non-Mageia systems
2. Mageia images can be created from within Mock
3. ARM images can be created from x86_64 hosts
4. Image building can rely on pre-installed repo configurations
This also brings us in line with the Docker image creation process,
which also uses DNF.
