Handle non-existing *generic kernel and initrd

Finding the Debian boot kernel and initrd is collapsed into a
single statement so that the script doesn't bail out when
'set -o pipefail' is set and the *generic kernel and initrd are
absent.

Change-Id: Ifd616818f7387e6f102636c7cf186be6097f1d5c
Closes-Bug: 1335016
Move install bin from rpm-distro to yum

rpm-distro element was trying to install bin programs into the
image but it doesn't provide any. Move that logic into the yum
element which provides the install-packages bin.

Change-Id: Id5339fc7ffbef6327fe41f50a3e39ab30b6d0320
Merge "Handle non-existing *generic kernel and initrd"
Check for epel before installing it

instack runs this script on a system where epel has already been
installed, which causes the yum command to fail.  Only installing
the epel-release rpm when it isn't already present addresses the
issue.

Change-Id: Ia179d7c7ed69ea5f785b5dcd16394c7663a145cc
Merge "Move install bin from rpm-distro to yum"
'iso' element to build bootable ISO images.

This commit adds a new element named 'iso' to build a bootable
ISO image for the kernel/ramdisk emitted by the 'baremetal' or
'ramdisk' element.

Change-Id: I89d175a29e2d0bc64b47fe527f0d0f6875f6849a