Add extra_dhcp_opts extension to Janus plugin.

This commit is for integrating Testbed with Ironic. In particular,
it instructs Janus to specify boot parameters when creating a network
port.

Ported from https://review.openstack.org/#/c/44564/
Merge pull request #8 from hongbin/ironic

Add extra_dhcp_opts extension to Janus plugin.