There are follow changes in the current patch:
- Update .gitignore to exclude releasenotes/build subdirectory.
- Add placeholders for notes and source files.
- Add index.rst with reference on unreleased notes.
- Use pbr to search branches and tags of project.
- Update tox.ini by new environment for building releasenotes.
Change-Id: I7f834a3fb05bf1a7d7565ce7bdb44a1d4b4e2e3d
da2833fa1f