Add memcached_servers to [DEFAULT]

In nova.conf, we specify memcached_servers but under the
[keystone_authtoken] section.  Since we run multiple nova-consoleauth
processes we need to utilise memcached, and this means adding an
entry for memcached_servers under [DEFAULT].

Addresses https://github.com/rcbops/ansible-lxc-rpc/issues/52.