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]
@rupakg I actually want to rename this method to some different altogether -- it really shouldn't have the `get_` prefix on it. I'm going to save that for another time though since we'll likely be changing this interface as we try to integrate other service providers.