Get Devstack to install app-catalog-ui by default

Have had conversations with a few different people and it sounds like the only way this would happen would be if the app-catalog-ui work was integrated directly in Horizon. I think that approach makes a lot of sense, and suggest we discuss in detail with the Horizon team during the Austin summit.

kfox1111 - Still big -1 from me at this point. The horizon plugin's tied to the app site. We want to be able to upgrade it outside of the 6 month cadence. I think minimum though, we'd need to have a horizon blocking gate check on the plugin to ensure it always works. Also, horizon is pushing more and more core plugins outside of horizon into their own repos. Some of them will likely get defaulted on in devstack since they were before. This is something that will need to be discussed too.