Reduce the scope of yamllints coverage

Since I76bc62ef22d09e1b6c19ebcd9b12f6381e1fd2db the non-voting linters
target has been failign as it was also checking yaml files in the
virtualenvs created by tox [1].  This change modifies the way we invoke
yamllint so that only files we actually care about are linted.

We also configure yamllint to have a slight more generous line-length
limit and to disable the 'truthy'[2] rule.

[1] http://logs.openstack.org/10/382110/2/check/gate-governance-linters-ubuntu-xenial/6c8ae21/console.html#_2016-10-11_15_39_52_575891
[2] https://yamllint.readthedocs.io/en/latest/rules.html#module-yamllint.rules.truthy

Change-Id: Ia0337aa49efb9ec832356eed374cc56bac1e75bc

.yamllint

@@ -0,0 +1,8 @@
+---
+rules:
+  line-length:
+    # Allow some of our longer URLs
+    max: 90
+    allow-non-breakable-words: yes           # Default
+    allow-non-breakable-inline-mappings: no  # Default
+  truthy: disable

tox.ini

@@ -10,7 +10,11 @@ setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:linters]
-commands = yamllint .
+whitelist_externals = bash
+commands = bash -c "find {toxinidir}        \
+            \( -name .tox -prune \)         \
+            -o -type f -name '*.yaml'       \
+            -print0 | xargs -0 yamllint"
            {toxinidir}/tools/check_projects_yaml_alphabetized.sh
 
 [testenv:venv]