Add a tasks resource for tracking long running actions for OpenStack Compute (nova)

Start of discussion: https://etherpad.openstack.org/p/IcehouseTaskAPI I'm proposing that in the v3 API we create a task for POST requests on instances, including the create request. One of the challenges in the API up to this point has been signalling a failure of an action without affecting the instance itself. ...

Improve cells in Nova/devstack so that it can be used for a gating check for OpenStack Compute (nova)

There are a few issues with running a gate job with cells that need to be addressed. 1. Currently even basic actions like booting an instance fail. Most likely devstack needs some work to configure cells properly. 2. Cells does not support all nova features. Picking a subset of devstack exercises and/or tempest te...