Allow users to configure Solr container's SecurityContext

[gerlowskija]

An alternate approach to #489.

#702 attempts to solve the same underlying issue by giving the Solr container in our STS template a hardcoded SecurityContext. But that effort is, at the time of writing, stalled out: partly due to questions about whether the hardcoded SecurityContext would be too restrictive for some usecases, and partly due to an inability to test in the relevant environments.

This PR takes a different approach by leaving the securityContext unset by default, and instead giving interested users the ability to specify an arbitrary securityContext as a part of their SolrCloud (or as a 'solr' helm chart setting). A securityContext can be provided in the resource YAML at .spec.customSolrKubeOptions.podOptions.containerSecurityContext (or using the podOptions.containerSecurityContext variable in the helm chart).

e.g.

apiVersion: solr.apache.org/v1beta1
kind: SolrCloud
metadata:
    ...
spec:
  solrImage:
    repository: solr
  customSolrKubeOptions:
    podOptions:
      containerSecurityContext:
        capabilities:
          drop:
            - ALL
        allowPrivilegeEscalation: false
  ...
  zookeeperRef:
    provided:
      chroot: "/"
      replicas: 3
      maxUnavailableReplicas: 1