qa/tasks: pass a hostname to "host add"

if orchestrator_cli uses cephadm as its backend, it will try to talk to
the remote host using ssh. but before this change, apparently, neither
"osd0" nor "mon0" is resolvable. that why we have

```
2020-01-29T22:49:16.187 INFO:tasks.ceph.mgr.x.smithi136.stderr:ssh: Could not resolve hostname osd0: Name or service not known
```

in the teuthology.log. and remoto throws an `execnet.gateway_bootstrap.HostNotFound`
exception in this case.

after this change, we always add localhost using "host add $hostname".

Signed-off-by: Kefu Chai <kchai@redhat.com>

qa/tasks/mgr/test_cephadm_orchestrator.py

@@ -16,8 +16,8 @@ def setUp(self):
         self._orch_cmd("set", "backend", "cephadm")
 
     def test_host_ls(self):
-        self._orch_cmd("host", "add", "osd0")
-        self._orch_cmd("host", "add", "mon0")
-        ret = self._orch_cmd("host", "ls")
-        self.assertIn("osd0", ret)
-        self.assertIn("mon0", ret)
+        remote = self.mgr_cluster.mon_manager.find_remote('osd', 1)
+        hostname = remote.hostname
+        self._orch_cmd("host", "add", hostname)
+        hosts = self._orch_cmd("host", "ls")
+        self.assertIn(hostname, hosts)