Introduce cinder_scheduler_default_filters

Operators may want to define specific default filters. Previously,
they could only do so via cinder_cinder_conf_overrides.

Make the cinder.conf template understand cinder_scheduler_default_filters,
a list variable that, if defined, is folded into a comma-separated
list for scheduler_default_filters in the [DEFAULT] section.

Change-Id: I12859167d19b9f40e3378ac08fed094a42f40bc7

releasenotes/notes/configurable-scheduler-filters-17ea2ed9d4aa0708.yaml

@@ -0,0 +1,25 @@
+---
+features:
+  - |
+    The list of enabled filters for the Cinder scheduler,
+    `scheduler_default_filters` in `cinder.conf`, could previously be
+    defined only via an entry in ``cinder_cinder_conf_overrides``. You now
+    have the option to instead define a list variable,
+    ``cinder_scheduler_default_filters``, that defines the enabled
+    filters. This is helpful if you either want to disable one of the
+    filters enabled by default (at the time of writing, these are
+    `AvailabilityZoneFilter`, `CapacityFilter`, and
+    `CapabilitiesFilter`), or if conversely you want to add a filter
+    that is normally not enabled, such as `DifferentBackendFilter` or
+    `InstanceLocalityFilter`.
+
+    For example, to enable the `InstanceLocalityFilter` in addition to
+    the normally enabled scheduler filters, use the following variable.
+
+    .. code-block:: yaml
+
+      cinder_scheduler_default_filters:
+        - AvailabilityZoneFilter
+        - CapacityFilter
+        - CapabilitiesFilter
+        - InstanceLocalityFilter

templates/cinder.conf.j2

@@ -17,6 +17,12 @@ auth_strategy = {{ cinder_auth_strategy }}
 ## Cinder API's enabled
 {% if cinder_services['cinder-volume']['group'] in group_names %}
 {% if cinder_service_backup_program_enabled == true %}
+
+## Cinder Scheduler
+{% if cinder_scheduler_default_filters is defined %}
+scheduler_default_filters={{ cinder_scheduler_default_filters | join(',') }}
+{% endif %}
+
 ## Cinder Backup
 backup_driver = {{ cinder_service_backup_driver }}
 backup_metadata_version = {{ cinder_service_backup_metadata_version }}