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].
Merge pull request #55 from mattt416/add_memcached_servers_to_default

Add memcached_servers to [DEFAULT]