Add a policy to decided instance start sequence

As some of the instances require a more stringent performance,usually we achieve a more reasonable performance requirements through human intervention and other measures at boot time. But,when we re-power the host, instances boot process become disorderly,often breaking the topology combination we plan earlier,resulting in decreased performance.

For example，we can set a priority value in ‘extra_specs’ attribute in flavor.
When start all the instances in a host,we can compare the flavor priority between two instances to decided which one to start earlier.
If priority values are equality,we can use the 'vcpu' attribute in instance.
When they all equality,we think they have the same priority to start.