Make installation of epel-release optional

The epel-release package provides a yum repository configuration for
EPEL, and points to the upstream EPEL mirrors. In some cases this is
undesirable, such as when a local EPEL mirror is available, or access to
the public repositories is not possible.

This change makes it possible to skip installation of epel-release, by
setting 'yum_install_epel' to 'false'.

Change-Id: Ib685b0c3e21df01c4dd177771f019fae4bb90e66
Story: 2003277
Task: 24218