Implements blueprint embrane-neutron-plugin
This commit implements the main Embrane plugin, which covers
route and extratoute extensions.
The plugin will rely on existing plugins for leveraging L2 networks,
for now it only supports OpenVSwitch and vlan networks, but more
supports are to come if the model is approved (or a different one
is suggested).
Change-Id: I90aff8ec4324bd3a7c59c411374db6a118d1a72b
d4c3047ba4