Remove deprecated config options 'resources_prefix'

Config option 'resources_prefix' was deprecated. It is
used by only manila plugin which fix has been commited.

This commit removes this config option and hardcode the
resource prefix as 'tempest'

Change-Id: Idce7af97fe6647e3095b2b626fbacb3446694fbc
Depends-On: I9c140b3a9efabeced7fc26cb1f169ebe6789d88b
diff --git a/releasenotes/notes/removal-deprecated-config-options-3db535b979fe3509.yaml b/releasenotes/notes/removal-deprecated-config-options-3db535b979fe3509.yaml
index 2b3bb3a..0b03e21 100644
--- a/releasenotes/notes/removal-deprecated-config-options-3db535b979fe3509.yaml
+++ b/releasenotes/notes/removal-deprecated-config-options-3db535b979fe3509.yaml
@@ -7,3 +7,4 @@
 
     * ``[identity-feature-enabled].forbid_global_implied_dsr``
     * ``[image-feature-enabled].deactivate_image``
+    * ``[default].resources_prefix``