feat: Add RHEL/CentOS gather_mac_addresses support
feat: Add RHEL/CentOS gather_mac_addresses support

The 'gather_mac_addresses.yml' playbook discovers the MAC addresses
associated with the corresponding network interfaces. RHEL and CentOS
default to an IPv6 configuration when generating traffic through the
switch in order to facilitate MAC address discovery. This commit adds
the necessary conditionals to prevent running Ubuntu specific steps on
RHEL or CentOS systems.

This commit also includes style changes to keep all lines less than 80
columns.

Change-Id: I6325ecce746fdb62bec1a892a0298fc5b7fa2d7c

open-power-ref-design-toolkit/cluster-genesis,open-power-ref-design-toolkit/cluster-genesis