Make user value for GlanceImageImportPlugin prevail on logic

We used to *append* image_conversion to the list of enabled Glance
import plugins when both Glance and Nova were configured to use RBD
but we should instead make user input prevail on the logic, assuming
users who customize the THT param know what they are doing and
eventually include image_conversion only when they need it.

Change-Id: I5c0788bb361323f5a0e69e22d4bd42fa3c1e06ed
(cherry picked from commit fe4253102a)

deployment/glance/glance-api-container-puppet.yaml

@@ -135,8 +135,11 @@ parameters:
       Whether to show multiple image locations e.g for copy-on-write support on
       RBD or Netapp backends. Potential security risk, see glance.conf for more information.
     type: boolean
+  # We default import plugins list to 'no_op' (instead of empty list) to discern from the scenario
+  # in which the user purposely disabled all plugins setting it to an empty list. This is useful
+  # to automatically enable image_conversion plugin only when value is left to the default.
   GlanceImageImportPlugins:
-    default: []
+    default: ['no_op']
     description: >
       List of enabled Image Import Plugins. Valid values in the list are
       'image_conversion', 'inject_metadata', 'no_op'.
@@ -301,9 +304,10 @@ conditions:
           expression: $.data.values().any($.get("GlanceBackend", "") = "rbd")
           data: {get_param: GlanceMultistoreConfig}
       - true
-  enable_image_conversion:
+  force_image_conversion_plugin:
     and:
       - rbd_backend_enabled
+      - equals: [{get_param: GlanceImageImportPlugins}, ['no_op']]
       - equals: [{get_param: NovaEnableRbdBackend}, true]
   use_tls_proxy: {equals : [{get_param: EnableInternalTLS}, true]}
   glance_workers_unset: {equals : [{get_param: GlanceWorkers}, '']}
@@ -409,7 +413,7 @@ outputs:
             glance::api::node_staging_uri: {get_param: GlanceNodeStagingUri}
             glance::api::image_import_plugins:
               if:
-              - enable_image_conversion
+              - force_image_conversion_plugin
               - list_concat_unique:
                 - {get_param: GlanceImageImportPlugins}
                 - ['image_conversion']