Persist scheduler hints for vm instance

When creating VMs with scheduler hints, scheduler hints will only take effect at deploying time. After the VM was deployed, scheduler hints will be lost.

Later on when someone try to migrate the VM, this VM can be migrated to a host which might violated the original scheduler hints. Same problems also exist for resize, cold migration etc.

A proposed solution was store scheduler hints so that it can be available for the whole life cycle of the VM instance, this can make sure the VM can retrieve and evaluate the scheduler hints before doing some VM operations so as to make sure the VM will always obey its scheduler hints during its life cycle.

This blueprint was mainly for persisting the scheduler hints to nova instance database and also enable "nova show" can list the scheduler hints.