Prepare filters for using RequestSpec object

Now that the FilterScheduler is using a RequestSpec object,
we need to change the filters for backporting the object
into the legacy dicts before changing each of them.

Release Notes will be updated to mention that custom filters
need to be modified to either use a RequestSpec object or
primitive them into a dict.

Partially-Implements: blueprint request-spec-object-mitaka

UpgradeImpact - see the reno file attached.

Change-Id: I14f18c4507498b1d24a9318fafc97193861ca0b6

releasenotes/notes/filters_use_reqspec-9f92b9c0ead76093.yaml

@@ -0,0 +1,9 @@
+---
+upgrade:
+  - |
+    Filters internal interface changed using now the RequestSpec NovaObject
+    instead of an old filter_properties dictionary.
+    In case you run out-of-tree filters, you need to modify the host_passes()
+    method to accept a new RequestSpec object and modify the filter internals
+    to use that new object. You can see other in-tree filters for getting the
+    logic or ask for help in #openstack-nova IRC channel.