Add cache=swift.cache for authtoken example.

- Things swill go badly with swift if we leave the default to authtoken
  to use its own memcache cache connection based python-memcache c based
  binding.

Change-Id: I293b875acdcb06e5a7a0cfa9a9bb5d7678675da0

doc/source/overview_auth.rst

@@ -96,6 +96,7 @@ add the configuration for the authtoken middleware::
   admin_tenant_name = service
   admin_user = swift
   admin_password = password
+  cache = swift.cache
 
 The actual values for these variables will need to be set depending on
 your situation.  For more information, please refer to the Keystone

etc/proxy-server.conf-sample

@@ -153,6 +153,7 @@ user_test_tester3 = testing3
 # admin_user = swift
 # admin_password = password
 # delay_auth_decision = 1
+# cache = swift.cache
 #
 # [filter:keystoneauth]
 # use = egg:swift#keystoneauth