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
OS-483, mutiple members for second network
Merge pull request #14 from Vegasq/OS-483

OS-483, mutiple members for second network
OS-479, DHCP service workflow fixes

    * dhcp_members was asigned but infoblox services wasnt restarted
    * dns server wasnt assigned in infoblox DHCP service
Merge pull request #13 from Vegasq/OS-479

OS-479, DHCP service workflow fixes
OS-432 Update existing NIOS network on subnet create

In case
 a. user creates a subnet for shared or external network
 b. NIOS network with the same CIDR exists
Update existing network, rather than raising an error.