Private node handle for topics and services

[akgoins]

The o3d3xx_node uses a public node handle to create all publishers and to get all parameters. If there are multiple cameras, each of these topics/parameters have to be remapped which results in a lot of excess remapping. It would be nice to have all of the topics and parameters use a private node handle to facilitate using multiple cameras at once without the need for lots of remapping.

[tpanzarella]

Leveraging the ns and nn args to the launch file automates all the remappings for you.