mdraid.py lib: Check if /usr/sbin/mdadm exists

Praviously the check was implemented using OSError return from `run`
function. However, in this particular case it's not safe and leads
to unexpected behaviour. Check the existence of the file explicitly
instead prior the `run` function is called.

Update existing unit-tests and extend the test case when mdadm
is not installed.

repos/system_upgrade/common/libraries/mdraid.py

@@ -1,3 +1,5 @@
+import os
+
 from leapp.libraries.stdlib import api, CalledProcessError, run
 
 
@@ -12,11 +14,13 @@ def is_mdraid_dev(dev):
     :raises CalledProcessError: If an error occurred
     """
     fail_msg = 'Could not check if device "{}" is an md device: {}'
+    if not os.path.exists('/usr/sbin/mdadm'):
+        api.current_logger().warning(fail_msg.format(
+            dev, '/usr/sbin/mdadm is not installed.'
+        ))
+        return False
     try:
         result = run(['mdadm', '--query', dev])
-    except OSError as err:
-        api.current_logger().warning(fail_msg.format(dev, err))
-        return False
     except CalledProcessError as err:
         err.message = fail_msg.format(dev, err)
         raise  # let the calling actor handle the exception

repos/system_upgrade/common/libraries/tests/test_mdraid.py

@@ -51,6 +51,7 @@ def test_is_mdraid_dev(monkeypatch, dev, expected):
     run_mocked = RunMocked()
     monkeypatch.setattr(mdraid, 'run', run_mocked)
     monkeypatch.setattr(api, 'current_logger', logger_mocked())
+    monkeypatch.setattr(os.path, 'exists', lambda dummy: True)
 
     result = mdraid.is_mdraid_dev(dev)
     assert mdraid.run.called == 1
@@ -62,6 +63,7 @@ def test_is_mdraid_dev_error(monkeypatch):
     run_mocked = RunMocked(raise_err=True)
     monkeypatch.setattr(mdraid, 'run', run_mocked)
     monkeypatch.setattr(api, 'current_logger', logger_mocked())
+    monkeypatch.setattr(os.path, 'exists', lambda dummy: True)
 
     with pytest.raises(CalledProcessError) as err:
         mdraid.is_mdraid_dev(MD_DEVICE)
@@ -71,6 +73,18 @@ def test_is_mdraid_dev_error(monkeypatch):
     assert expect_msg in err.value.message
 
 
+def test_is_mdraid_dev_notool(monkeypatch):
+    run_mocked = RunMocked(raise_err=True)
+    monkeypatch.setattr(mdraid, 'run', run_mocked)
+    monkeypatch.setattr(api, 'current_logger', logger_mocked())
+    monkeypatch.setattr(os.path, 'exists', lambda dummy: False)
+
+    result = mdraid.is_mdraid_dev(MD_DEVICE)
+    assert not result
+    assert not mdraid.run.called
+    assert api.current_logger.warnmsg
+
+
 def test_get_component_devices_ok(monkeypatch):
     run_mocked = RunMocked()
     monkeypatch.setattr(mdraid, 'run', run_mocked)