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().
try/except looks like unnecessary complication of the execution flow. I would suggest leaving `plugin = importutils.import_module(driver)` only.