Add a doc and test for data_utils.rand_password

This commit adds a documentation and a test about the length
parameter to data_utils.rand_password to clarify the behavior.

Change-Id: I26c7380d1430f107c0aaf705d160d8782b850438

tempest_lib/common/utils/data_utils.py

@@ -61,6 +61,7 @@ def rand_password(length=15):
     """Generate a random password
 
     :param int length: The length of password that you expect to set
+                       (If it's smaller than 3, it's same as 3.)
     :return: a random password. The format is
              '<random upper letter>-<random number>-<random special character>
               -<random ascii letters or digit characters or special symbols>'

tempest_lib/tests/common/utils/test_data_utils.py

@@ -66,10 +66,19 @@ class TestDataUtils(base.TestCase):
     def test_rand_password_with_len(self):
         actual = data_utils.rand_password(8)
         self.assertIsInstance(actual, str)
+        self.assertEqual(len(actual), 8)
         self.assertRegexpMatches(actual, "[A-Za-z0-9~!@#$%^&*_=+]{8}")
         actual2 = data_utils.rand_password(8)
         self.assertNotEqual(actual, actual2)
 
+    def test_rand_password_with_len_2(self):
+        actual = data_utils.rand_password(2)
+        self.assertIsInstance(actual, str)
+        self.assertEqual(len(actual), 3)
+        self.assertRegexpMatches(actual, "[A-Za-z0-9~!@#$%^&*_=+]{3}")
+        actual2 = data_utils.rand_password(2)
+        self.assertNotEqual(actual, actual2)
+
     def test_rand_url(self):
         actual = data_utils.rand_url()
         self.assertIsInstance(actual, str)