Extracted config check from pep8 to separate env

Pep8 is broken after each oslo update. Extracted config check to
a separate env. Next step - add this env to Jenkins (or sahara-ci)
as non-voting job once it is merged.

Change-Id: I58940e9606d736412c2c653ddb4ad4d09e6edcea
Closes-Bug: #1408053

tox.ini

@@ -30,7 +30,6 @@ downloadcache = ~/cache/pip
 [testenv:pep8]
 commands =
     flake8 {posargs}
-    {toxinidir}/tools/config/check_uptodate.sh
     # Check that .po and .pot files are valid:
     bash -c "find sahara -type f -regex '.*\.pot?' -print0|xargs -0 -n 1 msgfmt --check-format -o /dev/null"
     # Run bashate checks
@@ -63,6 +62,9 @@ commands =
     oslo-config-generator --config-file tools/config/config-generator.sahara.conf \
     --output-file etc/sahara/sahara.conf.sample
 
+[testenv:checkconfig]
+commands =
+    {toxinidir}/tools/config/check_uptodate.sh
 
 [testenv:pip-missing-reqs]
 # do not install test-requirements as that will pollute the virtualenv for