Enable to set DHCP port attributes

DHCP ports are created automatically. There is no chance to specify its attributes (ex. ip address) by a user.
There is a requirement to be able to specify ip address of a DHCP port by our customer.
 I propose adding a function to "add network to dhcp agent" API that enbale to specify a dhcp port attributes and create a dhcp port when the network attached to an agent.

API:
POST v2.0/agent/<agent id>/networks
body
{ 'network_id': <network id>,
  'dhcp_port': {'name': <name>, 'fixed_ips': <fixed_ips>, 'mac_address': <mac address>}}

'dhcp_port' is added. It is not required. If it is ommited a dhcp port will not be created at the time. There is no problem for API compatibility.