Currently most OpenStack code is linted, but YAML files are not. As a
result, sometimes YAML problems enter the code base (e.g., the key
duplicate fixed in change I7f2369adfb152fd2a74b9b105e969e653e592922).
This patch enables YAML linting in tox linting tests, using the
yamllint tool [1]. It checks syntax errors, key duplicates, and
cosmetic problems.
[1]: http://yamllint.readthedocs.org/
EDIT: It also fixes six errors (including key duplicates) that entered
the code base after the first fix change was merged.
(I7f2369adfb152fd2a74b9b105e969e653e592922)
Change-Id: Ie746230f28fe3ed0cf218201d5a3810f7bc44070
e9d9662bb6