Change memcache config entry name in Keystone to be consistent with Oslo

Currently, Keystone-Client is using 'memcache_servers' config option to store
the memcache server info, while in OSLO project, it is using 'memcached_servers'
to config the same option. It is better to change keystone-client's
'memcache_servers' to 'memcached_servers' to keep consistent between these two
projects.

Change-Id: I93ca0aa368f95a3ccf6de6984262057e61f75ffe
Fixes: Bug 1172793