Boot filename per client architecture

[domcleal]

Add a parameter to set per-architecture (boot) filenames in the DHCP config, e.g.

option architecture code 93 = unsigned integer 16 ;
if option architecture = 00:00 {
  filename "pxelinux.0";
} elsif option architecture = 00:06 {
  filename "shim.efi";
} else {
  filename "mypxefilename";
}

In theforeman/dhcp, a $bootfiles hash has filenames keyed by architecture codes: theforeman/puppet-dhcp@6d01407