[Sonarqube] Introduce extra mounts for install-plugins init container

[jordanjean]

This PR makes it possible to add volume mounts in the init container retrieving and installing plugins. It is especially useful to mount CA certificates in the container so that it is able to retrieve plugins from a repository using private PKI certificate, for instance:

persistence:
  volumes:  # Value already available
  - name: my-ca-trust
    hostPath:
      path: "/etc/ssl/certs"

plugins:
  mounts:
  - name: my-ca-trust
    mountPath: /etc/ssl/certs
    readOnly: true