Move networking from share manager to driver interface

Several things are implemented:
- Allocation/deallocation now handled by drivers instead of share manager.
It provides flexibility for drivers.
- Network plugin interface was updated to support new approach for
configuration options setting.

Config opts for network plugin can be defined via three sources:
a) using separate config group
b) using config group of back end
c) using DEFAULT config group
Variants (a) and (b) are mutually exclusive, there are switched by
opt 'network_config_group' that belongs to share driver interface.

Implements bp network-helper

Change-Id: I3b05369f01777675c1b834af5ee076d8b7219a0f
Merge "Move networking from share manager to driver interface"