Bump ansible for linters

I noticed this when I tried to use Ansible 2.5 "loop:" constructs.

Unfortunately, we can't just rely on bringing in zuul to pull the
right version of ansible.  ansible-lint being uncapped just takes it
over, and we end up with the latest version.  Pin it to the current
zuul requirements and add a note (maybe one day
https://github.com/pypa/pip/issues/988 will get a fix ...)

Change-Id: Iaf8f0f5cdc46df41fec7c436c1179c80bb5c368e

tox.ini

@@ -22,7 +22,10 @@ passenv =
 deps =
      # Zuul is required to supply the zuul ansible modules for ansible-lint
      -egit+https://git.openstack.org/openstack-infra/zuul#egg=zuul
-     ansible<2.4.0
+     # Unfortunately pip doesn't do actual dependency resolution, so
+     # we need to manually ansible at the top level to the same as
+     # zuul requirements.txt
+     ansible>=2.5.1,<2.6
      ansible-lint
      hacking>=0.10,<0.11
      bashate>=0.2