select_destinations returns a list of (host, node) tuples that satisfy
the request_spec and filter_properties passed to it.  This will allow
the conductor to get a list of destinations for workflows such as
creating or resizing an instance and then handle the orchestration
itself.

part of bp query-scheduler

Change-Id: I1a42cea64dbad67562d7efe8d759e6efb5ec8121

This method is required by the scheduler.manager.prep_resize method
which is being moved to conductor.manager.

We move it to scheduler.utils until we completely move migrations
logic out of scheduler.manager.

Partially implements bp cold-migrations-to-conductor

Change-Id: I8203577fc26da5cc1039815e31fb8fe356d1f7d6

Before moving the control of live-migration into the conductor,
extract the live-migration control logic into a separate class.

The callback to select_hosts will be replaced by a new
scheduler rpc method in a later changeset.

Part of blueprint live-migration-to-conductor
Change-Id: I6de33ada6dc377e20f8df07da92244f2c150b9fe

Implements blueprint per-aggregate-resource-ratio

* AggregateCoreFilter to support per-aggregate cpu_allocation_ratio
* Falls back to global setting if per-aggregate value not found

DocImpact

Change-Id: I9230f46e2490226f3c50d616aa173d4722095087

Implements blueprint per-aggregate-resource-ratio

* AggregateRamFilter to support per-aggregate ram_allocation_ratio
* Falls back to global setting if per-aggregate value not found

DocImpact

Change-Id: I93e069f0ac3f595d9a2a17119274c7bfbe8fca31

Make sure that when scheduling, the instance_type used in filters
contains the 'extra_specs'.  This is a bit ugly, but will get cleaned up
with objects.

Fixes bug 1192331

Change-Id: I3614f3a858840c9561b4e618fc30f3d3ae5ac689