Configure host and service dependencies

When adding a unit to a model, the nagios config should be updated with the correct dependencies either for hosts or services. This would greatly reduce the amount of alerts generated when a physical host with many containers goes down.

For example a host dependency could be:

define hostdependency {
        host_name PHYSICAL_HOST
        dependent_host_name LXD_CONTAINER
        notification_failure_criteria d,u
}