scheduler: Change servicegroup API is_up() command to accept host and topic (partial)

Currently, the servicegroup API's is_up() API call, which is used to determine if a service is up or down, accepts a dictionary of service information. With the detach-service-from-compute-node blueprint, it became clear that the is_up() API call should really just accept a host and a topic parameter, not a dictionary of information.

The reason for this is because the servicegroup API's current is_up() API call was built with various assumptions that the driver for the API was a database. For ZooKeeper and Memcache servicegroup drivers, there is no need to pass a dictionary of service information. We should only supply a host and a topic key for the service and the drivers themselves should be free to implement whatever underlying data model makes sense.