Accept multiple hosts when calling live migration

Currently calling the live_migrate API accepts a single hostname as the target for the live migration, or no hostname, in which case the scheduler picks a target. This blueprint is for expanding this API to optionally accept a list of hostnames in addition to zero or one. When a list of hostnames is passed, Nova will attempt to live migrate the instance to the first hostname on the list, and in the event of a failure, try each successive host in order until either the live migration succeeds, or the list is exhausted. The motivation for this change is the blueprint that will have Nova check the validity of the specified target with the scheduler to make sure that any original requirements for the instance are respected in the migrated instance, such as host aggregates, etc. (https://blueprints.launchpad.net/nova/+spec/check-destination-on-migrations). Since the target host may be rejected now, it will be more efficient to provide a list of hostnames rather than continuously call the API in a try/fail loop.