Add check for redundant import aliases

This adds a pep8 function that will check for redundant import aliases.
Any imports of the forms below will not be allowed.

from x import y as y
import x as x
import x.y as y

Change-Id: Iff90f0172d97bd1d49d54c811a70c8af11776da4

HACKING.rst

@@ -66,6 +66,7 @@ Nova Specific Commandments
 - [N357] Use oslo_utils.uuidutils or uuidsentinel(in case of test cases) to
   generate UUID instead of uuid4().
 - [N358] Return must always be followed by a space when returning a value.
+- [N359] Check for redundant import aliases.
 
 Creating Unit Tests
 -------------------