Network manager refactoring

It is a 1st step.

1. All class methods are @classmethods now and instantiating of class objects is removed.
2. There is a part independent from network type, Nova and Neutron parts are independent from each other.
3. Got rid from some unused code.

To be continued.

Implements: blueprint nailgun-network-manager-refactoring

Change-Id: I8d61a9acae995ff3e61f100a1d26dcc77df014a6