DHCP: add mac address formatting to the template

Cobbler allows users to enter mac addresses in a number of formats,
but DHCP requires that it be in unix format. Updated the dhcp
template to format the mac address using netaddr EUI dialect=mac_unix

Signed-off-by: Tomo Berry <tbberry@us.ibm.com>
Merge pull request #1362 from tbberry/master

DHCP: add mac address formatting to the template