live-migration failed after re-add nova-compute unit

[Xiang Hui]

[ENV]
trusty + icehouse

[DESCRIPTION]
If you re-add a nova-compute unit with the following steps, live-migration will fail, this is found at Maas + OpenStack, because it seems Maas will reinstall the system on nova-compute node when it's re-added.

[STEP]
2 compute-nodes, node-a and node-b.

1. Create an instance(ex. created on node-a)
2. live-migrate the instance to node-b
$ nova live-migrate <instance-id> node-b
3. after migration has completed, live-migrate back to node-a
$ nova live-migrate <instance-id> node-a
4. delete the nova-compute unit which is node-b
$ juju delete-unit nova-compute/1
5. add the nova-compute unit again to node-b
$ juju add-unit nova-compute
6. live-migrate once again to node-b
$ nova live-migrate <instance-id> node-b
7. check if the host has changed, however nothing changed and no error
$ nova show <instance-id>
8. from the log file in compute-node node-a, an error was shown.

Error message on compute-node:

2015-04-20 06:45:21.206 11564 ERROR nova.virt.libvirt.driver [-] [instance: 79fc6061-66cc-46b8-9df5-0d002fd8c4d6] Live Migration failure: operation failed: Failed to connect to remote libvirt URI qemu+ssh://node-b/system: Cannot recv data: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
9a:4c:37:23:f3:c3:1d:3c:24:f0:2a:46:be:65:ca:c7.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending RSA key in /root/.ssh/known_hosts:8
remove with: ssh-keygen -f "/root/.ssh/known_hosts" -R node-b
RSA host key for node-b has changed and you have requested strict checking.
Host key verification failed.: Connection reset by peer