Add a sims build triggered by the lsst_distrib weekly build

The sims team would like an automatically built weekly sims version that is built on the weekly build of lsst_distrib. They would like the result to be a binary distribution that would allow end users to install a full stack of sims plus DM dependencies that are all consistent with the weekly build. I.e.

$> eups distrib install lsst_sims -t w_2018_13

This is currently possible to do by hand, but it would be useful to them to have it automatically build. They are very clear that a failing lsst_sims weekly build should not imply a failing lsst_distrib build.