secret initialization via init-container

dep.yml

      initContainers:
      - name: init-config
        image: quay.io/prometheus/alertmanager:v0.12.0
        env:
        - name: SMTP_AUTH_USERNAME
          valueFrom:
            secretKeyRef:
              name: alert-email-secret
              key: username
        - name: SMTP_AUTH_PASSWORD
          valueFrom:
            secretKeyRef:
              name: alert-email-secret
              key: password
        command:
        - sh
        - -c
        - sed -i -e "s/{{SMTP_AUTH_USERNAME}}/$(SMTP_AUTH_USERNAME)/g" /etc/alertmanager/alertmanager.yml && sed -i -e "s/{{SMTP_AUTH_PASSWORD}}/$(SMTP_AUTH_PASSWORD)/g" /etc/alertmanager/alertmanager.yml
        volumeMounts:
        - name: config-volume
          mountPath: /etc/alertmanager