IpsetManager refactoring

Refactor the IpsetManager to move all the low level
knowledge about the ipset behaviour and performance
considerations from the firewall driver to the manager,
reduce redundant function names, and change missleading
variables talking about ipset chains, where they
should say ipset sets.

No logical changes to behaviour, just responsibilities
moved from one class to another.

Unit testing is move from iptables_firewall to ipset_manager
to test the new responsibilities of the class.

Implements: blueprint ipset-manager-refactor

Change-Id: I93000a37a71cd22753b32edbd0a5f3c9cb8b0bde
Merge "IpsetManager refactoring"