Add notes on live migration configuration
Add notes on live migration configuration

Live migration will not currently work with a multi-node DevStack
environment due to the libvirtd process running as root and the compute
process trying to use the stack user's SSH keys with the default live
migration URI.

The multi-node documentation is updated to outline how to configure the
SSH keys between the source and destination stack and root users so that
live migration will work as expected.

Co-Authored-By: Taylor Peoples <4cd16e0a53bfc05843b4568218a3e60ed1698543@us.ibm.com>
Change-Id: Ifed7e95cd5ba43a05a4099a3664cbefde3e0a749
Closes-Bug: #1487262

noironetworks/devstack,Mahito/devstack,dtroyer/devstack,bigswitch/devstack,bigswitch/devstack,richliu/devstack,richliu/devstack,bswartz/devstack,noironetworks/devstack,group-policy/devstack,samgoon/devstack,Mahito/devstack,group-policy/devstack,dirkmueller/devstack,dtroyer/devstack,nawawi/openstack,bswartz/devstack,nawawi/openstack,dirkmueller/devstack,openstack-dev/devstack,openstack-dev/devstack,samgoon/devstack