Comment 1 for bug 1391896

how to reproduce:

1) insert 'exit 1' before `umount` https://github.com/stackforge/fuel-main/blob/master/image/ubuntu/create_separate_images.sh#L136
2) run 'make target_ubuntu_image' (it's faster than full iso rebuilding)
3) wait until it get failed on that line.
4) 'make deep_clean' will leave ubuntu mirror and the root fs image mounted. Run build again and it will produce this err http://jenkins-product.srt.mirantis.net:8080/job/6.0.iso/94/console
5) manually umount ubuntu mirror. 'make deep_clean' again and build then will produce this err http://jenkins-product.srt.mirantis.net:8080/job/6.0.iso/96/console