Accept $NBCACHE_PORT as a memcache URL

When linking to the memcached container, environment variables are populated with connection info. Example:

```
core@nbviewer ~ $ /usr/bin/docker run --rm --name nbviewer.1.service -P --link nbcache:nbcache ipython/nbviewer env
HOME=/
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=47013ea59ae8
NBCACHE_PORT=tcp://172.17.0.8:11211
NBCACHE_PORT_11211_TCP=tcp://172.17.0.8:11211
NBCACHE_PORT_11211_TCP_ADDR=172.17.0.8
NBCACHE_PORT_11211_TCP_PORT=11211
NBCACHE_PORT_11211_TCP_PROTO=tcp
NBCACHE_NAME=/nbviewer.1.service/nbcache
NBCACHE_ENV_LANGUAGE=en_US.UTF-8
NBCACHE_ENV_LANG=en_US.UTF-8
NBCACHE_ENV_LC_ALL=en_US.UTF-8
LANGUAGE=en_US.UTF-8
LANG=en_US.UTF-8
LC_ALL=en_US.UTF-8
NBVIEWER_THREADS=2
```

This just handles `NBCACHE_PORT` directly so we don't have to hack around to get the server list into `MEMCACHE_SERVERS`.