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
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