Create external_service parameter

[dhollinger]

Added an external_service boolean parameter for allowing puppet-rsyslog to manage configs/logs shared with other processes that may be managed by other modules.

The most common example would be if someone is using garethr-docker to create and manage an rsyslog docker container and crayfishx-rsyslog to manage configuration that is shared with the container via a volume.

In this example, garethr-docker generates a service file (upstart, init.d, or systemd) that will stop/start the container as though it were a service. As rsyslog requires a stop/start to read configuration changes, allowing this module to set an externally managed service that crayfishx-rsyslog can notify on configuration changes is not only extremely helpful, but almost required.

external_service is a boolean that defaults to false, MUST be used in conjunction with service_name, and CANNOT be used with manage_service.