Unit test fails in pbuilder environment

When running tests inside a pbuilder environment we don't
have /dev/disk/by-path directory and so the test_host_driver
test fails. Fix test by overriding the isdir method to
return True and pretend that the directory is there.

Change-Id: Ifd39d796b7f250b1ec45227a4262ebb33d5656bf

cinder/tests/brick/test_brick_connector.py

@@ -105,6 +105,7 @@ class HostDriverTestCase(test.TestCase):
 
     def setUp(self):
         super(HostDriverTestCase, self).setUp()
+        self.stubs.Set(os.path, 'isdir', lambda x: True)
         self.devlist = ['device1', 'device2']
         self.stubs.Set(os, 'listdir', lambda x: self.devlist)