Adding display_group parameter to Linode instance provider

While working with the Linode instance provider in Ansible, I found it useful to assign a display_group dynamically to the instance which was created so that the corresponding Linode inventory provider could report it as a group to which plays could be assigned.

This pull request adds it as a parameter that may be passed to this provider and has been successfully tested it against several new Linode instances.

Let me know if I can answer any questions or address any concerns, and thanks for reading it over!