Adding support for OpenShift securityContext

[computate]

Adding an openshift: true boolean field to the operator to remove the default Solr fsGroup: 8983 securityContext used in Kubernetes.

apiVersion: solr.apache.org/v1beta1
kind: SolrCloud
spec:
  customSolrKubeOptions:
    podOptions:
      openshift: true

I have successfully tested these changes in OpenShift Local and confirmed that an empty securityContext allows the solrcloud pod run on OpenShift.

spec:
  template:
    spec:
      securityContext: {}

Fixes #466