Implementing MAXMEMORYPOLICY for redis images

[cdchris12]

This PR implements the ability to configure redis' maxmemory-policy setting via the new MAXMEMORYPOLICY variable.

Partially fulfills #476

[tobybellwood]

can confirm:

docker run --rm -it -e MAXMEMORY=200mb -e MAXMEMORYPOLICY=allkeys-lfu testlagoon/redis-5:pr-477
...
redis:/data$ printenv | grep MAXMEMORY
MAXMEMORYPOLICY=allkeys-lfu
MAXMEMORY=200mb
redis:/data$ redis-cli config get maxmemory*
1) "maxmemory"
2) "209715200"
3) "maxmemory-samples"
4) "5"
5) "maxmemory-policy"
6) "allkeys-lfu"