Run doc8 first
Run doc8 first

For "tox -e docs", run doc8 first.

doc8 runs much faster than sphinx builds and
will provide quick feedback on syntax issues.

Change-Id: Id4f77020a70acd8eb9df513d43f86d9fe78f7e22

j-griffith/cinder,phenoxim/cinder,phenoxim/cinder,Datera/cinder,mahak/cinder,Datera/cinder,openstack/cinder,j-griffith/cinder,mahak/cinder,openstack/cinder