Allow redis config from Pillar data; maxmemory setting.

Redis without bounds will chew up all the monitoring server memory if there is a major Logstash backlog to process. This adds the ability to limit memory via the maxmemory setting.

In the process, I've created pillar data overrides for common settings. 

Can we use the same pattern as for other projects. I.e. see:
https://github.com/ministryofjustice/postgresql-formula/blob/master/postgresql/map.jinja

Otherwise we will end up PRing every config parameter separately.