Instance affinity filters do not work in a heterogeneous cloud with Ironic computes

Upstream bug: https://bugs.launchpad.net/nova/+bug/1606496

Description
===========

In a heterogeneous cloud with both libvirt and ironic compute nodes instance affinity filters like DifferentHostFilter or SameHostFilter do not filter hosts out when scheduling a subsequent instance.

Steps to reproduce
==================

Make sure you have at least two libvirt compute nodes and one ironic node.

Make sure DifferentHostFilter and SameHostFilter are configured as nova-scheduler filters in nova.conf, filters scheduler is used.

1. Boot a libvirt instance A.
2. Check the host name of the compute node instance A is running on (nova show from an admin user).
3. Boot a libvirt instance B passing a different_host=$A.uuid hint for nova-scheduler.
4. Check the host name of the compute node instance B is running on (nova show from an admin user).

Expected result
===============

Instances A and B are running on two different compute nodes.

Actual result
=============

Instances A and B are running on the same compute node.

nova-scheduler logs shows that DifferentHost filter was run, but did not filter out one of the hosts: Filter DifferentHostFilter returned 2 host(s) get_filtered_objects

Environment
===========

MOS 9.1

2 libvirt compute nodes
1 ironic compute node
FiltersScheduler is used
DifferentHostFilter and SameHostFilter filters are enabled in nova.conf

Root cause analysis
===================

Debugging shown that IronicHostManager is configured to be used by nova-scheduler instead of the default host manager, when Ironic compute are deployed in the same cloud together with libvirt compute nodes.

IronicHostManager overrides the _get_instance_info() method and unconditionally returns an empty instance dict, even if this method is called for non-ironic computes of the same cloud. DifferentHostFilter and similar filters later use this info to find an intersection of a set of instances running on a libvirt compute node (currently, always {}) and a set of instances uuids passed as a hint for nova-scheduler, thus compute nodes are never filtered out and the hint is effectively ignored.