Flake8: Fix and enable H404

Fix and enable flake8 test H404:
H404  multi line docstring should start without a leading new line

Change-Id: I13dc13bb5e81d65300680df094cde538c7a6f6df
Partial-Bug: #1333290

tools/lintstack.py

@@ -71,8 +71,10 @@ class LintOutput(object):
 
     @classmethod
     def from_msg_to_dict(cls, msg):
-        """From the output of pylint msg, to a dict, where each key
-        is a unique error identifier, value is a list of LintOutput
+        """From the output of pylint msg, to a dict.
+
+        Each key is a unique error identifier, value is a list of
+        LintOutput.
         """
         result = {}
         for line in msg.splitlines():