Check the destination host when migrating (Newton)

== This is a follow-up blueprint of the Mitaka one ==

Migrations (incl. evacuate, live and cold migrate) allow the possibility to either specify a destination host or not. The former option totally bypasses the scheduler by calling the destination Compute RPC API directly.
Unfortunately, there are some cases when migrating a VM, it breaks the scheduler rules so it makes some further boot requests potentially broken due to some constraints not enforced when migrating (like allocation ratios)
As the current API method called select_destinations() only returns one destination per instance requested, there is no way to know if the destination host was a valid target.

The blueprint will accordingly expose a new Scheduler RPC API method called check_my_destination() which will accept filter_properties and request_spec as regular arguments, plus the name of the compute node and will return True or False.
The API for all migrations will be modified to add an extra request body argument called 'check' (accepting True or False) and the corresponding CLI methods will expose that with --check