Allow v3 identity to work without the admin domain name

The problem was that the value of admin_domain_name was required in order to
use identity v3 even if no admin was being used.

A new option auth.default_credentials_domain_name
is used instead of admin_domain_name except when requesting admin creds.
This defaults to 'Default' which is the name keystone uses for compatibility
with v2.
Because tenant_isolation and pre-provisioned credentials are mutually
exclusive, and to avoid having too many config options, the new option is
used instead of tenant_isolation_domain_name as well.

Change-Id: I52f0d4c0cc7e5eafa896776b12315ed6154dfae2
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 724bff4..1f8c889 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -123,10 +123,10 @@
 # Roles to assign to all users created by tempest (list value)
 #tempest_roles =
 
-# Only applicable when identity.auth_version is v3.Domain within which
-# isolated credentials are provisioned.The default "None" means that
-# the domain from theadmin user is used instead. (string value)
-#tenant_isolation_domain_name = <None>
+# Default domain used when getting v3 credentials. This is the name
+# keystone uses for v2 compatibility. (string value)
+# Deprecated group/name - [auth]/tenant_isolation_domain_name
+#default_credentials_domain_name = Default
 
 # If allow_tenant_isolation is set to True and Neutron is enabled
 # Tempest will try to create a useable network, subnet, and router