Remove CONF.volume_feature_enabled.bootable

The option is deprecated already and the os-set_bootable feature has
been implemented since Ib6cdca15950fe86b4fb2a43cfc7338e28260e453 .
That was 2.5 years ago. So we can remove this option.

Change-Id: I83085bc9e9747a5f427d1651408b265cc1e2030e
diff --git a/releasenotes/notes/remove-bootable-option-024f8944c056a3e0.yaml b/releasenotes/notes/remove-bootable-option-024f8944c056a3e0.yaml
new file mode 100644
index 0000000..e6e53af
--- /dev/null
+++ b/releasenotes/notes/remove-bootable-option-024f8944c056a3e0.yaml
@@ -0,0 +1,6 @@
+---
+deprecations:
+  - The *bootable* config option in the *volume_feature_enabled* group is
+    removed because the corresponding feature os-set_bootable has been
+    implemented 2.5 years ago and all OpenStack versions which are supported
+    by Tempest should support the feature.