Provide a good API for all instance actions

Whle http://developer.openstack.org/api-ref-compute-v2.1.html#os-server-actions-add-v2.1 already provides a good way for knowing what happens with a Request ID, it's actually missing a few things :
 - not all the actions actually update the instance_actions table (so they are not provided to the API)
 - sometimes a Task ID would be nice for querying some long-time actions
 - some new features could be nice like cancelling the current action, or having a task progress etc.

to be clear, we are not opiniating whether os-server-actions is the best REST resource or if it should be called os-tasks, that separate discussion should be done during the spec review.

This BP aims to ressurect the previous https://blueprints.launchpad.net/nova/+spec/instance-tasks-api but first working on fixing the instance actions API rather than directly providing a new Tasks API.

Old bits can be found in https://etherpad.openstack.org/p/midcycle-instance-tasks and https://review.openstack.org/#/q/topic:bp/instance-tasks-api,n,z but they are not planned to be restored now.