Revert "fix zaza.model.run_on_unit" (#277)

Reverts #257

Since the issue
[#649](openstack-charmers/zaza#649) was fixed.

tests/functional/tests/model.py

@@ -58,8 +58,7 @@ def test_scp_from_unit(self):
         test_file = "test.txt"
         path = f"/tmp/{test_file}"
         exp_path = os.path.join(tmp_dir, test_file)
-        zaza.sync_wrapper(self.model.connect)()
-        zaza.sync_wrapper(self.model._model.units[TESTED_UNIT].run)(f"echo 'test' > {path}")
+        zaza.model.run_on_unit(unit_name=TESTED_UNIT, command=f"echo 'test' > {path}")
 
         zaza.sync_wrapper(self.model.scp_from_unit)(TESTED_UNIT, path, tmp_dir)
         self.assertTrue(os.path.exists(exp_path))