Allow overriding the  scheduler_host_manager

Deployments with Ironic need a custom host manager. Allow overriding it.

Don't add PciPassthroughFilter explicitly to the active filters.
nova.scheduler.filters.all_filters should include this if it's in the
default filters, and if it is removed from scheduler_default_filters in
the pillar, the new scheduler_custom_filters will allow users to
manually add it.
diff --git a/README.rst b/README.rst
index cc6f92b..39df559 100644
--- a/README.rst
+++ b/README.rst
@@ -399,6 +399,18 @@
   glance image-update --property hw_scsi_model=virtio-scsi <image>
   glance image-update --property hw_disk_bus=scsi <image>
 
+Scheduler Host Manager
+----------------------
+
+Specify a custom host manager.
+
+.. code-block:: yaml
+
+  nova:
+    controller:
+      scheduler_host_manager: ironic_host_manager
+
+
 Documentation and Bugs
 ======================