Make the tox docs env build the docs

The tools/build_docs script builds the docs for devstack. Use it.

Change-Id: I530ff6c8090f2c58160cb11bee35534b79db52ed
Move the doc paths to match other repos

In order to build in the Infra system, we'd like to be able to reuse the
existing doc build macros. To support that, move docs/source to
doc/source and docs/html to doc/build/html.

Change-Id: Ibd8e8e82e54c69b182120df67e6ec6908fed2908