helm-chart: improve redis

[wanddynosios]

We'd like to use podinfo as a demo app in a hardened cluster (pss restricted; network-policy deny-all by default).

Also, we'd like to demo persistence in a simple use-case with redis.

#345 addresses these points:

• adds securityContext for redis
• adds network policies (redis <-> podinfo)
• adds pvc for redis

[wanddynosios]

@stefanprodan Hi, can I do something to get this merged? :-)