updated per Steve Langasek's feedback:

- put umount in trap
- wrap comment lines at 78 chars
- remove unnecessary DEBIAN_FRONTENT env var from apt-get lines

live-build/ubuntu-cpc/hooks/042-vagrant.binary

@@ -3,16 +3,17 @@
 #
 # Generate a generic Vagrant Box.
 #
-# Vagrant images are essentially nothing more than OVA's with extra-metadata and
-# some preinstalled packages.
+# Vagrant images are essentially nothing more than OVA's with extra-metadata
+# and some preinstalled packages.
 #
-# We can't use the OVA's for Vagrant since Vagrant uses SSH to modify the instance.
-# This build step creates a cloud-config ISO so that Cloud-Init will configure
-# the initial user, creates meta-data that tells Vagrant how to interact with
-# the cloud-init created users, and finally create the OVA.
+# We can't use the OVA's for Vagrant since Vagrant uses SSH to modify the
+# instance. This build step creates a cloud-config ISO so that Cloud-Init
+# will configure the initial user, creates meta-data that tells Vagrant how
+# to interact with the cloud-init created users, and finally create the OVA.
 #
-# For this step, we make a deriviative of binary/boot/disk.ext4 and install some
-# packages in it, convert it to a vmdk, and then assemble the vagrant box.
+# For this step, we make a deriviative of binary/boot/disk.ext4 and install
+# some packages in it, convert it to a vmdk, and then assemble the vagrant
+# box.
 
 cur_d=${PWD}
 my_d=$(dirname $(readlink -f ${0}))
@@ -33,18 +34,23 @@ apt-get -qqy install genisoimage qemu-utils
 box_d=$(mktemp -d)
 seed_d=$(mktemp -d)
 mount_d=$(mktemp -d)
-trap "rm -rf ${box_d} ${seed_d} ${mount_d}" EXIT
+
+create_derivative "disk" "vagrant" #sets ${derivative_img}
+mount_disk_image ${derivative_img} ${mount_d}
+
+cleanup_vagrant() {
+    umount_disk_image ${mount_d}
+    rm -rf ${box_d} ${seed_d} ${mount_d} ${derivative_img}
+}
+trap cleanup_vagrant EXIT
 
 pkgs=(virtualbox-guest-dkms
       virtualbox-guest-utils
 )
 
-create_derivative "disk" "vagrant" #sets ${derivative_img}
-mount_disk_image ${derivative_img} ${mount_d}
-env DEBIAN_FRONTEND=noninteractive chroot ${mount_d} apt-get update
-env DEBIAN_FRONTEND=noninteractive chroot ${mount_d} apt-get install --no-install-recommends -y ${pkgs[@]}
-env DEBIAN_FRONTEND=noninteractive chroot ${mount_d} apt-get clean
-umount_disk_image ${mount_d}
+chroot ${mount_d} apt-get update
+chroot ${mount_d} apt-get install --no-install-recommends -y ${pkgs[@]}
+chroot ${mount_d} apt-get clean
 
 # Used to identify bits
 suite=$(chroot chroot lsb_release -c -s)
@@ -55,7 +61,6 @@ distro=$(chroot chroot lsb_release --id --short | tr [:upper:] [:lower:])
 prefix="${distro}-${suite}-${version}-cloudimg"
 vmdk_f="${box_d}/${prefix}.vmdk"
 create_vmdk ${derivative_img} ${vmdk_f}
-rm -f ${derivative_img}
 
 # Vagrant needs a base user. We either inject the well-known SSH key
 # or use password authentication. Both are ugly. So we'll use a password