Refactoring of the pipeline processing and state transitions

[giffels]

Currently the state transistions between internal states are defined by nested dictionaries. For example:

tardis/tardis/resources/dronestates.py

Lines 94 to 105 in 63e1865

	transition = {
	ResourceStatus.Running: lambda: {
	MachineStatus.NotAvailable: lambda: IntegratingState(),
	MachineStatus.Available: lambda: AvailableState(),
	MachineStatus.Draining: lambda: DrainingState(),
	MachineStatus.Drained: lambda: DisintegrateState()
	},
	ResourceStatus.Booting: lambda: defaultdict(lambda: BootingState),
	ResourceStatus.Deleted: lambda: defaultdict(lambda: DownState),
	ResourceStatus.Stopped: lambda: defaultdict(lambda: CleanupState),
	ResourceStatus.Error: lambda: defaultdict(lambda: CleanupState)
	}

In addition, a pipeline of function calls is used to decided the next state of a resource. For example:

tardis/tardis/resources/dronestates.py

Line 107 in 63e1865

processing_pipeline = [resource_status, batchsystem_machine_status]

This representation, in particular the nested dictionaries are confusing and should be change by a different representation.