Implement drivers for core plugins

Implements plugable way to get network information from L2 plugins.
Depending on core_plugin appropriate facade module gets loaded.
Currently contains drivers for ml2 and openvswitch.
Includes tests for new code.
OS-522 Rename init_l2() and Facade class

- Rename init_l2 to init_driver
- Rename Facade class to Driver class
- Fixed several comments
UT and manual tests passed fine with this patch
Merge pull request #12 from bondar-pavel/os-522

OS 522 Rework NetworkL2InfoProvider in plugable way