ipa_backup.py: replace /var/lib/ipa/backup with paths.IPA_BACKUP_DIR
/var/lib/ipa/backup is defined in ipaplatform.paths as paths.IPA_BACKUP_DIR
Remove all instances of /var/lib/ipa/backup/ in ipa_backup.py.
Signed-off-by: François Cami <fcami@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>