updated install and kickstart scripts for priorities

Until yum-plugin-priorities is moved to a product channel, the following
env variables need to be set to make sure priorities are honored:
CONF_OPTIONAL_REPO=1
CONF_RHEL_OPTIONAL_REPO=<repo base>/6Server/x86_64/optional

CONF_RHEL_OPTIONAL_REPO is only needed if using the yum install method