Clean up Open vSwitch configuration for Quantum NEC plugin.

* Set datapath_id before connecting to the OpenFlow controller.
  Previously datapath_id is changed after connecting to the controller.
* Drop "0x" prefix from datapath_id passed to OVS.
* Fix a bug that SKIP_OVS_BRIDGE_SETUP skips all confiugration of
  the plugin agent. It should skip only OVS setup.