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: Ia1d5e5ac8d3da58f1fe1db68763f7431e1506b13

openstack/ironic,openstack/ironic