Remove docker registry from prometheus image stream

During testing, we need to use released prometheus image, but currently the prometheus image stream uses the same registry as all other syndesis stuff:

name: '{{.Syndesis.Spec.Registry}}/{{.Images.PrometheusImagePrefix}}/{{ .Images.Support.Prometheus }}:{{ .Syndesis.Spec.Components.Prometheus.Tag }}'

Removing .Syndesis.Spec.Registry and prepending PrometheusImagePrefix in config.yaml with docker registry should help (the same as with oauth proxy):

Images:
  OAuthProxyImagePrefix: quay.io/openshift
  PrometheusImagePrefix: <add registry here as in quay>prom