SOLR StatefulSet gets a broken securityContext by default

Upon deploying a SolrCloud CR for the first time with the Operator (version 0.6.0), ZK is fine but the SOLR StatefulSet fails to start Pods because of:
```
create Pod demo-solrcloud-0 in StatefulSet demo-solrcloud failed error: pods "demo-solrcloud-0" is forbidden: unable to validate against any security context constraint: [provider restricted: .spec.securityContext.fsGroup: Invalid value: []int64{8983}: 8983 is not an allowed group provider restrictedfsgroup: .spec.securityContext.fsGroup: Invalid value: []int64{8983}: 8983 is not an allowed group]
```

If in the SolrCloud CR I specify an empty context, everything start fine:
```yaml
spec:
customSolrKubeOptions:
podOptions:
podSecurityContext: {}
```

I don't know if this is expected, a bug or some edge case for OpenShift but there was no documentation for it.

Start by deploying the version 0.6.0 SolrCloud example on OpenShift and inspect the generated SOLR StatefulSet securityContext under the restricted security context constraint. Compare that behavior with the podOptions.podSecurityContext: {} workaround; done means a default SolrCloud deployment creates Pods successfully without requiring that override.

Written by the indexing model from the issue text.