De-dupe dex client configuration, assign dex secret

[richard-cox]

Dex integration was added by #167.

This includes the ability to disable dex and also provide a secure dex client key. However this has led two issues

Helm dex configuration is duplicated

The epinio chart values now contains duplicate sections for dex and epinio-ui: dex (see below for example)

Dex client secret

The dex client contains a secret that is used in it's chart template. This same value must also be used in the epinio-ui chart. This is currently duplicated, but should change to use a kube secret anyway

Example epinio values.yaml

global:
  domain: '<snip>'
dex:
  enabled: true
  ui:
    secret: 'dev-secret1'
epinio-ui:
  epinioUI:
    apiSkipSSL: true
  dex:
    enabled: true
    ui:
      secret: "dev-secret1"

[richard-cox]

I've moved dex.enabled out to global.dex.enabled in order for sub chart to make use of it.

I've also used a kube secret for the dex ui client secret

epinio/helm-charts@c4cf206