Add memcached_backend option in keystone.conf

[Kui Shi]

If we choose memcached as the token backend driver, we should set proper servers in '[memcache]' section of keystone.conf.

For the memcache backend, there are 3 memcached client supported by dogpile,
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# pydoc dogpile.cache.backends.memcached

   dogpile.cache.api.CacheBackend(__builtin__.object)
        GenericMemcachedBackend
            BMemcachedBackend
            MemcachedBackend(MemcacheArgs, GenericMemcachedBackend)
            PylibmcBackend(MemcacheArgs, GenericMemcachedBackend)

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In keystone, they are listed in keystone/common/kvs/backends/memcached.py:

VALID_DOGPILE_BACKENDS = dict(pylibmc=memcached.PylibmcBackend,
                              bmemcached=memcached.BMemcachedBackend,
                              memcached=memcached.MemcachedBackend)

By default, memcached will be used. The "memcached_backend" option will determine which backend to use.
But there is no simple to pass the "memcached_backend" option.

[Morgan Fainberg (mdrnstm)]

The idea behind the base memcache token driver is that it makes some basic decisions for you. If you would like to utilize a more in-depth dogpile configuration, ideally you would do the raw "kvs" configuration passing extra dogpile config options to the backend (it's not user friendly to do so).

This change would be good making the UX better when configuring the use of memcached as a token persistence store.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Dolph Mathews (<email address hidden>) on branch: master
Review: https://review.openstack.org/122037
Reason: Abandoning due to inactivity and failing tests.

[Dolph Mathews (dolph)]

I've abandoned the referenced changed due to inactivity and failing tests. If this is still an issue, please reset the status.

[Morgan Fainberg (mdrnstm)]

Was incomplete ages ago, inactive and generally not needed now with oslo_cache etc.