The ndisc6 package is not yet available in EPEL 8.
See: https://bugzilla.redhat.com/show_bug.cgi?id=1779134
Until the package is available set the pkg-map to "" for
the ndisc6 package when distro is redhat and install the
package using || true in the element script instead so
that CentOS 8 build's do not fail because of the missing
package.
Once the package is in EPEL 8 this change can be reverted.
Related-Bug: #1754219
Change-Id: Icd4bad8852ce5ba40fb0e7b0d335191efbe88c67
7948fee7e2