Minor: remove unnecessary intermediate variable

Removes an unnecessary intermediary variable and an
unnecessary list extend operation that implied previous
list members where there weren't any. There should be no
functional change. This just improves readability slightly.

Change-Id: Ice412c29be083d82e055cc0bc45ed8b97c7628d0
Fix KeyError in dhcp_rpc when plugin.port_update raise exception

KeyError exception is seen because of following reasons

* DhcpRpcCallback._port_action() is called by two functions
   -  DhcpRpcCallback.create_dchp_port()
   -  DhcpRpcCallback.update_dhcp_port()

* When create_dhcp_port() function calls _port_action(), the
  function argument 'port' will have the body as
  {'port': {'network_id': foo_network_id, 'fixed_ips': [..] ...}

* When update_dhcp_port() function calls _port_action(), the
  function argument 'port' will have the body as
  {'id': port_id, 'port': {{'port': {'network_id': foo_network_id,
			            'fixed_ips': [..] ...}}

* If an exception occurs when _port_action() calls plugin.create_port(),
  network id is accessed as
  net_id = port['port']['network_id']

* If an exception occurs when _port_action() calls plugin.update_port(),
  network id is accessed as
  net_id = port['port']['network_id']
  which is causing the KeyError. network_id should have been accessed as
  net_id = port['port']['port']['network_id']

This patch fixes the issue by making the _port_action() take the
same port body. update_dhcp_port() insteading of passing the port_id
and port information in a single argument, it now adds port_id
in the port body itself.

Change-Id: I70b92fa20b421b05ca2053a9a57f62db726f7625
Closes-bug: #1378508
Merge "Fix KeyError in dhcp_rpc when plugin.port_update raise exception"
Merge "Minor: remove unnecessary intermediate variable"