Add a latex command to our sphinx builders.
Add a latex command to our sphinx builders.

The libra project added this locally and use it for some docs they make. It
seemed like something we could add what we provide globally.

Change-Id: I219829693568362bbe01354a8f477a10031754c4

JioCloud/oslo-incubator,JioCloud/oslo-incubator,JioCloud/oslo-incubator