Add a config option for trusts

This commit adds a new config section identity-feature-enabled which
contains the options to select which optional/configurable features in
keystone are enabled. The only option that fits this category at this
time is trusts.

Partially implements bp config-cleanup

Change-Id: I8110156e48616534c45e9eb2c12557b2d6bdfd46
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 2173eb6..4c2cdd7 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -428,6 +428,17 @@
 #admin_password=pass
 
 
+[identity-feature-enabled]
+
+#
+# Options defined in tempest.config
+#
+
+# Does the identity service have delegation and impersonation
+# enabled (boolean value)
+#trust=true
+
+
 [image]
 
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