Integrate w/ fleet-api gem

Removes all of the Fleet integration code from the panamax_agent and uses the fleet-api gem instead.

The biggest change is in the way that the service definition is submitted to the Fleet gem. Previously, we were using a model object to represent the unit file. In the new implementation the unit file is simply a Ruby hash.

[finishes #77159442]
@bdehamer Do you think the `get_state` should now be called `get_status` to be consistent. I know that may trigger a lot of upstream changes, but I did not find the UI project directly using that call.