Redis password should be a global config in k8s

[hardbyte]

Following this guidance, we now use a global.postgresql section so subcharts can access the postgres credentials without duplicating information. With redis we currently have to have the same password twice which isn't ideal:

redis:
  ## Note the `server` and `use_sentinel` options are ignored if provisioning redis
  ## using this chart.

  ## External redis server url/ip
  server: ""

  ## Does the external redis server support the sentinel protocol?
  use_sentinel: false

  ## Note if deploying redis-ha you MUST have the same password below!
  password: "9X9RFrRnQHcB"

redis-ha:
  ## Settings for configuration of a provisioned redis ha cluster.
  ## https://github.com/helm/charts/tree/master/stable/redis-ha
  ## Provisioning is controlled in the `provision` section
  auth: true
  redisPassword: "9X9RFrRnQHcB"

Instead we should have a global.redis section in our default values.yaml file.