DHCP global search by record + pxegrub option

DHCP global search by record (MAC, IP or hostname). Example:
- <proxy-host>:8443/dhcp/find/<MAC|IP|hostname>

DHCP option "pxegrub" added to allow to specify a path to pxegrub file
to override default menu.lst-<MAC>. Requires dhcpd.conf to have the
following 2 lines in it:

option grubmenu code 150 = text;
option dhcp-parameter-request-list = concat(option
dhcp-parameter-request-list,bootmenu);

Example:
curl -X POST http://dhcp:8443/dhcp/<network> -d 'hostname=blah' -d
'ip=<ip>' -d 'mac=<mac>' -d
'grubmenu=pxelinux.cfg/01-<mac-with-dashes>' -d 'filename=pxegrub.0'