Setup for openstack-doc-tools gating

Add files to allow us gating via:
tox -e checkbuild
tox -e checkniceness
etc

Enhance .gitignore to ignore files used by gating and by more
editors.

Change-Id: Id33d04f734d438d01dd5bbd784abaa213de5dbc9
Merge "Setup for openstack-doc-tools gating"