Support overriding image pull secrets on broker/proxy/pulsar-manager

[sijie]

[MarvinCai]

seems we're defining duplicated key, can we do:

      {{- if .Values.broker.imagePullSecrets }}
      imagePullSecrets:
      - name: {{ .Values.broker.imagePullSecrets }}
      {{- else }}
      {{- if .Values.imagePullSecrets }}
      imagePullSecrets:
      - name: {{ .Values.imagePullSecrets }}
      {{- end }}
      {{- end }}

[ericsyh]

Close legacy PR