enable local memcache for token caching

[James Page]

$ keystone-manage doctor

WARNING: `keystone.conf [cache] enabled` is not enabled.

    Caching greatly improves the performance of keystone, and it is highly
    recommended that you enable it.

We should install and enable a local memcache daemon on each keystone unit to improve performance for token validation.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/511465

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/511465
Committed: https://git.openstack.org/cgit/openstack/charm-keystone/commit/?id=4b00281bb495de71120e3fe6d27d269721d39aff
Submitter: Zuul
Branch: master

commit 4b00281bb495de71120e3fe6d27d269721d39aff
Author: Liam Young <email address hidden>
Date: Thu Oct 12 10:47:03 2017 +0000

    Add memcache backend

    Install and configure memcached on the keystone units and configure
    keystone to use the cache. This should speed up token access for
    existing tokens.

    Change-Id: I26af0a97660e5bbe293a32e6b9e3d209338f905a
    Closes-Bug: #1722541