'Get DHCP options for this instance.
Some hypervisors (such as bare metal) require that instances boot from
the network, and manage their own TFTP service. This requires passing
the appropriate options out to the DHCP service. Most hypervisors can
use the default implementation which returns None.
This is called during spawn_instance by the compute manager.
Note that the format of the return value is specific to the Neutron
client API.
:return: None, or a set of DHCP options, eg:
|    [{\'opt_name\': \'bootfile-name\',
|      \'opt_value\': \'/tftpboot/path/to/config\'},
|     {\'opt_name\': \'server-ip-address\',
|      \'opt_value\': \'1.2.3.4\'},
|     {\'opt_name\': \'tftp-server\',
|      \'opt_value\': \'1.2.3.4\'}'

def dhcp_options_for_instance(self, instance):

    return None
