image_rotate: clean up

[mikeferguson]

This is the first component/node with a cleanup pass to be fully implemented:

• Fix ros2 version: image_rotate does not initialize the target and source _vectors in constructor #740 by initializing vectors. Do this by declaring parameters AFTER we define the callback
• Implement lazy subscribers (I missed this in the earlier PRs)
• Add image_transport parameter so we can specify that desired transport of our subscriptions
• Update how we test for connectivity (and update the debug message - has nothing to do with whether we are remapped, it's really about whether we are connected)

[mikeferguson]

This debug message didn't work at all - made a more robust version by checking the overlap of our topic name with the actual topics that exist (as many of our depth_image_proc stuff does)

[mikeferguson]

this is unneeded - none of the parameters that are able to be updated would impact how the subscribers would actually be setup