Always cache tokens in a shared memcache

Instead of using in-process caching for tokens per service per
worker (disabled by default now), use a shared memcache to cache
token validation(s). This should both offload/speedup validations
and avoid the issues surrounding inconsistent validation responses
when using in-process caching [since each worker caches separately].

Change-Id: Ifc17c27744dac5ad55e84752ca6f68169c2f5a86
diff --git a/files/debs/keystone b/files/debs/keystone
index 370e4aa..fd0317b 100644
--- a/files/debs/keystone
+++ b/files/debs/keystone
@@ -1,5 +1,6 @@
 libkrb5-dev
 libldap2-dev
 libsasl2-dev
+memcached
 python-mysqldb
 sqlite3