Revert "Fixed use of flake8"

This reverts commit 74317a3445.

What I missed was that "dib-lint" is distributed and people use this
to lint their out-of-tree elements (e.g. project-config).  Because
dib-lint runs flake8 that really makes it a valid runtime dependency.

However, I don't think we need to pin to these particular versions of
flake8 in requirements (which I think was causing the original issues,
as it's fairly specific and easily conflicts).  It seems to make more
sense to use the current uncapped "hacking" versions; so move them
from test-requirements.txt.  Add a note so we don't forget again.

Change-Id: Ic2eee8f5b64c7020e98e0b1ef43a40f24411891a

requirements.txt

@@ -9,3 +9,5 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 PyYAML>=3.12 # MIT
 six>=1.10.0 # MIT
 stevedore>=1.20.0 # Apache-2.0
+# NOTE(ianw) in here because dib-lint uses flake8
+hacking<1.2.0,>=1.1.0

test-requirements.txt

@@ -2,7 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 
-hacking<1.2.0,>=1.1.0
 pylint==1.7.6;python_version=='2.7' # GPLv2
 pylint;python_version>'3.0' #GPLv2