Chore: Add sphinxcontrib-openapi for API documentation

Chore: Add sphinxcontrib-openapi for API documentation

Replace the swaggerv2doc directive usage in genericresource.rst with
the actively maintained sphinxcontrib-openapi extension.

Changes:
- Add sphinxcontrib-openapi==0.7.0 to requirements and conf.py
  (pinned to avoid sphinx-mdinclude/docutils incompatibility)
- Add POSIX-compatible download script to fetch OpenAPI specs during build
- Update genericresource.rst to use openapi directive
- Remove unnecessary OpenStack yoga constraints from docs tox.ini
- Retain sphinxcontrib-swaggerdoc module (removal in separate change)

Issue-ID: CIMAN-33
Change-Id: Ic4b1a5ae8740bce998cf179262e3c9beb8716e47
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>