Expose sudo permissions on the worker pool object

[mpdehaan]

In the security guide we should talk about putting the vespene_build user in the same group as the vespene user, and then we can expose "permissions_hex" on the worker pool DB object, allowing the build roots to be 770 vs wide open.

https://talk.msphere.io/t/i-cannot-get-a-build-to-work/107/7