neutron-ha-tool: Fix return values of agent_(alive|dead)_id_list

Both function are supposed to return a list of agent_ids not a list of agents.
Seems this got introduced by the recent code clean up (1ab0e472).

Change-Id: I3b4ef33deed7bff5f40787785077fb473b2bd0c3
Merge "neutron-ha-tool: Fix return values of agent_(alive|dead)_id_list"