Add roles to all tempest created users

Adds configuration option identity.tempest_roles which is a list of
roles that are added to all tempest created users.

This is useful when tempest users require priviledges between default
and admin.

Change-Id: I545d8f595dad6e4fe3da57e936baf18bc16c0e78
Closes-Bug: #1411256
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 2e1d6f3..234273b 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -582,6 +582,9 @@
 # applies to user and project (string value)
 #admin_domain_name = <None>
 
+# Roles to assign to all users created by tempest (list value)
+#tempest_roles =
+
 
 [identity-feature-enabled]