[RFE] Bare metal node N+1 redundancy

[Vladyslav Drok]

This proposes high availability/reliability feature to support Bare
Metal Node N+1 Redundancy based on the blueprints
volume-connection-information [1] and enhance-power-interface-for-soft-reboot-and-nmi [2].

When a bare metal node is failed due to hardware problem or is likely to be failed due to a sign of hardware failure, this function allows to switch over the node safely to another bare metal node in
short time if the failed node is booted from a share file system such as
FC, FCoE, iSCSI, NFS and etc.

This function exposes an API for switching over and would have an
Ironic CLI such as "ironic host-evacuate-cold" which was borrowed from
"nova host-evacuate-live".

[1] https://bugs.launchpad.net/ironic/+bug/1526231
[2] https://bugs.launchpad.net/ironic/+bug/1526226

- Nov 9th, 2015, Naohirot:
Overall idea can be found in the following presentation slides P.60 - P.63.

"Ironic Towards Truly Open and Reliable, Eventually for Mission Critical"
 http://www.slideshare.net/naohirot/ironic-towards-truly-open-and-reliable-eventually-for-mission-critical
 https://www.youtube.com/watch?v=MpSqDA3jo0I

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/259320

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Dmitry Tantsur <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/ironic-specs/+/259320
Reason: Abandoning because of inactivity