Use shared memcached instance, but set a Key prefix

Swift caching hashes its keys with md5, so the only thing needed
to avoid lookup collisions is to add a unique prefix to the nova
dashboard memcache keys. The 2nd instance of memcached needed
is difficult on platforms that use systemd/sysvinit for handling
services.

This reverts commit c216e76926ae9065747ea5455da4dfc59ef0c7ba.