Add pep8 check for tempest.lib import

tempest.lib should not import local tempest code to avoid circular dependency,
so this patch adds pep8 check to block such kind of code.

Change-Id: I392d28b3195040a800d96171ef275c6e73f9fef4
diff --git a/HACKING.rst b/HACKING.rst
index efabaf6..44519d4 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -19,6 +19,7 @@
          decorators.skip_because from tempest-lib
 - [T110] Check that service client names of GET should be consistent
 - [T111] Check that service client names of DELETE should be consistent
+- [T112] Check that tempest.lib should not import local tempest code
 - [N322] Method's default argument shouldn't be mutable
 
 Test Data/Configuration