isc-dhcp: allow hosts to have dhcp options

isc-dhcp: allow hosts to have dhcp options

Sometimes it's necessary to set per-host options like:

    list dhcp_option 'option:always-broadcast,true'

for hosts that don't understand unicast replies.  There might
be other options you might want to set on a per-host basis,
such as extensions-path, dhcp-message-text, etc.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>