automate ovs datapath configuration

- This change automates generation of
  the datapath_type paramater in the ml2_conf.ini
- If enable_ovs_dpdk is no the datapath type
  will be system else it will be netdev which maintains
  the current default behaviour when dpdk is not enabled.

Change-Id: Iedbf39c621f3c0237f9b9934492b12c600f2c4d3
partial-implementes: bp/ovs-dpdk