o-full: enable rhel-x86_64-server-lb-6 RHN channel

this channel is required for haproxy.
o-full: add keepalived
add keepalived
o-full: RHN stuff only on RedHatEnterpriseServer
o-full: fails if some RPM are missing

This should be done in install_packages instead to catch properly
the case where yum silently ignore some packages.
puppet-master: add_epel_repository only on RPM distro