OS 522 Rework NetworkL2InfoProvider in plugable way

Implements plugable way to get network information from L2 plugins.
Depending on core_plugin appropriate driver for gets loaded.
Currently contains drivers for ml2 and openvswitch.
Driver should implement method get_network_binding().
Few issues here that I see:
 # `DRIVER_PREFIX` and `DRIVER_SUFFIX` are local variables and don't need to be capitalized;
 # The logic for searching L2 backend is based on hardcoded string;
 # The class name for L2 facade is hardcoded and expected to be the same for all the backends, which is kind of weird.

I would suggest doing L2 plugin registration the other way around – L2 backend registers itself in the plugin framework with `register_l2_provider()` method.