Add support for executing cron in different container

By default, cron is executed on PHP containers. We should give an option to the user where he can run a cron in other site containers like Nginx or Postfix.

One such use case came to light when we wanted to implement logrotate on a server which required periodic execution on Nginx container(since logs were not mounted on the host then).

Although since now we can run the cron job on the host, still it would be great to have this capability