Add an api call to check that a target is reachable

[pglass]

I'm imagining an api call like GET /v1/{tenant}/targets/{target}/ping, which returns a 200 or 404 based on whether the target is reachable.

For a target with an ssh address, this would check that rift can successfully connect to that server via ssh. For a target with a nova address, this would check that rift can successfully fetch that server from the api.