Update bindep.txt for doc builds
Update bindep.txt for doc builds

https://governance.openstack.org/tc/reference/project-testing-interface.html#documentation
specifies that doc dependencies need the doc tag in bindep, use it.

Leave test environment in for transition period until we use the new
scripts.

Change-Id: I4a46d77036e6bbfd45b576ce8e0528d5660f188d

openstack/training-labs,openstack/training-labs,openstack/training-labs