add respective value to chart values file to support private repo with image pull secret

[dhorner71]

add imagePullSecrets and respective templating.

default:

imagePullSecrets: []

templating:

{{- with .Values.imagePullSecrets }}
imagePullSecrets:
  {{- toYaml . | nindent 2 }}
{{- end }}

example:

imagePullSecrets:
- name: image-pull-secret