cache: add support for memcached

[Invenio-Cache used to test the `memcached` backend](https://github.com/inveniosoftware/invenio-cache/blob/master/.travis.yml#L25). This is not possible anymore with `docker-services-cli` so it had to be [removed from the GA builds](https://github.com/inveniosoftware/automation-tools/pull/14).

## Describe the solution you'd like
```
$ docker-services-cli up --cache memcached
```

Note: some `memcached` config used to exist in this repo, but was removed as part of https://github.com/inveniosoftware/docker-services-cli/pull/19 since we were lacking the most important bit, having the service in the `docker-compose.yml` and implementing the health check.