clara images: Allow to change the name of the output image

Now 'clara images create', 'clara images repack' and 'clara images unpack'
will take an argument with the name of the image if we want to use something
different to the name by default.
clara images: if list_files_to_install is not present, do not try to install any file
temporarily disabled until we found out why we need this.
Add possibility to run a script after the image creation

The path to the script must be added in the config.ini
Update clara images unpack
clara images edit: show messages after creating the image so the user see them
clara images initrd: run mkinitramfs inside the target image
update config.ini
update changelog