Move DISKLAYOUT_FILE to default.conf? #678
Description
When looking at #668 I realized that DISKLAYOUT_FILE is defined in many places:
$ grep -r DISKLAYOUT_FILE= .
./usr/share/rear/layout/save/GNU/Linux/10_create_layout_file.sh:DISKLAYOUT_FILE=${DISKLAYOUT_FILE:-$VAR_DIR/layout/disklayout.conf}
./usr/share/rear/lib/checklayout-workflow.sh: DISKLAYOUT_FILE=$TEMP_LAYOUT
./usr/share/rear/lib/mkbackuponly-workflow.sh: DISKLAYOUT_FILE=$TMP_DIR/backuplayout.conf
./usr/share/rear/lib/savelayout-workflow.sh: DISKLAYOUT_FILE=$VAR_DIR/layout/disklayout.conf
But it is not defined in default.conf and the setting in savelayout-workflow.sh actually repeats the builtin default in 10_create_layout_file.sh.
I suspect that it would be better (more ReaR-like and less error prone) to have the default setting in default.conf.