Support migration and evacuation for Ironic driver

Currently, migrate() and evacuate() APIs are not supported in ironic driver.
This BP aims at implementing these two APIs in ironic driver.

IIUC, for VMs (such as libvirt driver),
- migrate: Migrate a VM from compute node A to compute node B.
- evacuate: Migrate all VMs on compute node A to other compute nodes.

So, migrate is an operation to a VM, and evacuate is an operation to a compute node.

To baremetals, there are two things we need to handle:
1. When a baremetal is broken, migrate it to another backup baremetal. This means use another baremetal to replace it. (migrate)
2. When a compute node is broken, evacuate all instances on that node to another compute node. (evacuate)

Correspondingly, two steps to implement these features mentioned above:
step-1. Add baremetal migration feature into nova to integrate the Ironic BP[1],
step-2. Improve the nova-evacuate to evacuate the failed baremetal compute node,
              it will be a long term plan.

This BP depends on the following Ironic BP.
[1]https://blueprints.launchpad.net/ironic/+spec/bare-metal-node-n+1-redundancy