Use global.imagePullSecrets at solr-operator helm chart

[j-coll]

In some scenarios, having to use pull secrets is mandatory. Even if using a private repo, or pulling from the public repos, but hitting the dockerhub anonymous pull limit (https://www.docker.com/increase-rate-limits).

The idea would be to use the global.imagePullSecrets, as in the main solr(cloud) helm, or in the zookeeper-operator.