health-check: Fixing package selection on RHEL

Many packages are not up to date or even doesn't exist in epel.
So let's specify the packages we want.
init.common: Enforcing path to include *sbin

When running it on RHEL, the /usr/sbin & /sbin was missing making many
commands failing because of out the default PATH
pxe: Fixing epel configuration

add_epel_repository expects to get a dist not a directory.