Implements operational status API for OVS plugin. Uses existing
database model for operational status. For network, status is always
up. For a port, status is down unless there is an agent that
indicates that the port is active on its compute host.
Updated: use Salvatore's config hook to set default op-status for OVS
in OVS run_tests.py
Updated: fixed issue in _make_port_dict() found by Brad.
Change-Id: I0560cbde9dc69bd4211d9aaf12cef204df2f7664