Allow disable redis

[ChenyuanZ]

This PR is to fix issue #304

Redis shipped in seldon-core helm chart is used for two purposes:

1. Store oauth keys for seldon-apiserver.
2. Persist Predictive Unit if its .s2i/environment PERSISTENCE flag is set to 1.

Since we can use ambassador to replace seldon-apiserver (which means we can disable seldon-apiserver in that case), and we can use a different Redis server for PERSISTENCE flag (if the admin already has a Redis server running), there is a need to disable the Redis shipped in seldon-core helm chart.

[ChenyuanZ]

Tested:

$ helm install seldon-core/helm-charts/seldon-core --name seldon-core --namespace czhang --set ambassador.enabled=true --set apife.enabled=false --set redis.enabled=false
NAME:   seldon-core
LAST DEPLOYED: Mon Nov 19 14:20:20 2018
NAMESPACE: czhang
STATUS: DEPLOYED

RESOURCES:
==> v1/ServiceAccount
NAME    SECRETS  AGE
seldon  1        0s

==> v1beta1/Role
NAME          AGE
seldon-local  0s
ambassador    0s

==> v1/RoleBinding
NAME    AGE
seldon  0s

==> v1/Pod(related)
NAME                                                 READY  STATUS             RESTARTS  AGE
seldon-core-ambassador-5cdff4c54c-kv7lr              0/1    ContainerCreating  0         0s
seldon-core-seldon-cluster-manager-75765b5cbd-kxpgh  0/1    ContainerCreating  0         0s

==> v1beta1/ClusterRole
NAME               AGE
seldon-crd-czhang  0s

==> v1/ClusterRoleBinding
NAME           AGE
seldon-czhang  0s

==> v1beta1/RoleBinding
NAME        AGE
ambassador  0s

==> v1/Service
NAME                          TYPE      CLUSTER-IP      EXTERNAL-IP  PORT(S)                     AGE
seldon-core-ambassador-admin  NodePort  10.100.158.174  <none>       8877:32532/TCP              0s
seldon-core-ambassador        NodePort  10.100.86.250   <none>       80:32221/TCP,443:31361/TCP  0s

==> v1beta1/Deployment
NAME                                DESIRED  CURRENT  UP-TO-DATE  AVAILABLE  AGE
seldon-core-ambassador              1        1        1           0          0s
seldon-core-seldon-cluster-manager  1        1        1           0          0s


NOTES:
Thank you for installing Seldon Core.

Documentation can be found at https://github.com/SeldonIO/seldon-core