Update to ansible 2.7 for linting

We run ansible 2.7 with zuul, so update the version we use for linting.

Change-Id: Ifdbd75272e525b499fe97cbf72e9d735fd13876e

tox.ini

@@ -32,7 +32,7 @@ deps =
      # 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>=2.7.0,<2.8
      ansible-lint<4
      hacking>=1.1,<1.2
      bashate>=0.2