Add API to access all VMs events/actions since the last access

Nova API should be extended to provide the information about events (and the corresponding actions) for all the instances. It is useful for gathering statistics about how much time instances worked, were paused or were in some other states and so on. Currently this information could also be accessed via InstanceAction API Extension for the specific instance, but not for all the instances.

The solution is to provide a separate API extension EventsLog (and the corresponding support on nova-client side). It should provide access to all nova events/actions, filtered by type since the the last access (the last viewed event is specified by event marker).