Avoid duplicate 'barbican' service_available option

The cinder and barbican tempest plugins both need the 'barbican'
service_available option to be defined, but they both can't register
the option. This patch ensures the cinder plugin registers the option
only when the barbican plugin isn't present.

Closes-Bug: #1991068
Change-Id: I51a22afae4fc98e2c2b8c1e82e8211a27649022c
diff --git a/cinder_tempest_plugin/config.py b/cinder_tempest_plugin/config.py
index 9cc3573..78dd6ea 100644
--- a/cinder_tempest_plugin/config.py
+++ b/cinder_tempest_plugin/config.py
@@ -26,11 +26,11 @@
 
 # The barbican service is discovered by config_tempest [1], and will appear
 # in the [service_available] group in tempest.conf. However, the 'barbican'
-# option isn't registered by tempest itself, and so we do it here. This adds
-# the ability to test CONF.service_available.barbican.
+# option isn't registered by tempest itself, and so we may need to do it.
+# This adds the ability to test CONF.service_available.barbican.
 #
 # [1] I96800a95f844ce7675d266e456e01620e63e347a
-service_available_option = [
+barbican_service_option = [
     cfg.BoolOpt('barbican',
                 default=False,
                 help="Whether or not barbican is expected to be available"),