Fixes 'ovs-agent cannot start on Windows because root_helper opt is not found'

Fixes 'ovs-agent cannot start on Windows because root_helper opt is not found'

Change Ie1224f8a1c17268cd7d1c474ed82fdfb8852eaa8 removed
config.register_root_helper() from ovs_neutron_agent:main, thus the agent
cannot start on Windows since root_helper config option is never
registered.

The fix consists in adding config.register_root_helper in the Windows side
utils.

Closes-Bug: #1497930

Change-Id: I6adc3b869c9564cac3ab99174b8fe47584213e59
(cherry picked from commit 074f82d3ec18fd591e9d1805676dfe13bd508510)