Update docs building

We need to move to new docs buidling structure as
per the Project Testing Interface (PTI) [0] for doc
build in OpenStack-Infra. More details [1].

[0] https://governance.openstack.org/tc/reference/project-testing-interface.html#documentation
[1] http://lists.openstack.org/pipermail/openstack-dev/2017-November/124815.html

Change-Id: I699095c2eb2c818354fe0502cbbbadd3b9ef2656
4 files changed