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().
After discussing, it turns out, that modifying L2 coreplugings to call register_l2_provider is not really good, because it is related only to infoblox specific code.
So when neutron ships without infoblox code, it will cause issues or unnecessary code complications for l2 coreplugins. 
I'll fix point 1, but 2 and 3 are left as is.