Move duplicated _get_node(s)_mac_addresses()

Move duplicated _get_node(s)_mac_addresses() from pxe and ssh drivers
to a common place.

Change-Id: I27d1df6d3b63f3421d59d14d2b5b9f2c874d6580
Merge "Move duplicated _get_node(s)_mac_addresses()"