Add support for Nova live migrations

This patch adds support for live migration in a new live_migrate_server
benchmark scenario. Live migrate requires an admin context, as
does determining the host on which an instance is running. A special
is_ready function was added to check whether the instance host
record changed. This is because live migrations can fail and simply
return the instance to an active state, so the only way to know for
sure we have completed successfully is to check the host before and
after the operation is initiated.

Change-Id: I642f0ecc2b5c84202ebacd01e2b0c85ab609e597