Create memcache_servers option

Permits use of one or more memcache servers for token storage.
Use to override default server (localhost:11211) when token_driver
is keystone.token.backends.memcache.Token.

Change-Id: Iaeb0c896d4907fd520db9b60e3af002b3711bc75
Merge "Create memcache_servers option"
Merge git://github.com/puppetlabs/puppetlabs-keystone