Refactored networker significantly to simplify it and improved testing and logging.

Summary of changes:
 * No more using global variables.
 * Using names.MachineTag internally, not a string.
 * No state split in 3 places and coupled in a way making it very hard to test (dropped state.go and implemented ConfigFile which knows how to read its data and render its own config).
 * No more reading and parsing network config files, just rewritting them with managed config as needed.
 * VLAN module loading is only done if needed.
 * Much better and simpler tests.
 * Live tested on MAAS and local (trusty, precise, utopic).
 * Disabling a physical interface causes any dependents to be disabled as well (i.e. VLANs and other virtual interfaces with the same MAC).
 * Rather than parsing (obsolete) network config files, the available network interfaces on the machine are discovered using the net package's functions (updates happen in real-time).
 * Much, much better logging (mostly on DEBUG level, but not always)
 * Not a simple NotifyWorker anymore, as it needs to do more things, i.e. implemented a full-scale worker loop, etc.
 * IMO much easier to read, understand and maintain the code after refactoring.
It is pre-existing state yes, by default network interfaces are created as enabled (IsDisabled field is false), but I agree shouldDisable is better name for that argument.