Allow excluding image type capabilities

During the implementation of blueprint request-filter-image-types,
I left a FIXME to follow-up and add an ability for an operator to
configure compute nodes to not expose image formats that they *can*
support, for situations where they may not be desired. Consider a
remote compute node which *can* support raw images, but for which
you want to require that the user use qcow2 for transfer efficiency.

This adds a new compute-level config option which allows excluding
image types, which will be filtered out of the traits list in a
general way, to avoid having to honor the list in each driver.

Change-Id: Iabe6246dba212b1a287a82cc0cf16e2e8c8a24b8

nova/conf/compute.py

@@ -979,6 +979,27 @@ Additional documentation is available here:
 
   https://docs.openstack.org/nova/latest/admin/managing-resource-providers.html
 
+"""),
+    cfg.ListOpt('image_type_exclude_list',
+                default=[],
+                help="""
+A list of image formats that should not be advertised as supported by this
+compute node.
+
+In some situations, it may be desirable to have a compute node
+refuse to support an expensive or complex image format. This factors into
+the decisions made by the scheduler about which compute node to select when
+booted with a given image.
+
+Possible values:
+
+* Any glance image ``disk_format`` name (i.e. ``raw``, ``qcow2``, etc)
+
+Related options:
+
+* ``[scheduler]query_placement_for_image_type_support`` - enables
+  filtering computes based on supported image types, which is required
+  to be enabled for this to take effect.
 """),
 ]