support ssh-based live migration

This change introduces support for ssh key generation and distribution
to support instance migration via qemu+ssh connections to remote
hypervisors.

This change requires https://review.openstack.org/#/c/89620/ (which
introduces support for ssh keys into puppet-nova)

Change-Id: I38c2aee3a7415bb69a099419f6149151c7d6b52c
Closes-bug: 1311168