Config updates and note for install_minimal.

I'm creating this pull request against saas, because that's where the config values below are required, however we would probably want to put this in 2.5.x since the way we install saas is by installing 2.5.x and switching the branch to saas.  When we install from 2.5.x, we'll be copying that branch's airtime.conf to /etc/airtime/conf, so by default you'd still need to manually edit the config file, which kind of defeats the purpose of this commit.

Is there any possible down side to putting these changes in 2.5.x and pulling them from there into saas and other branches?
We only have support for static_base_dir and station_id in one of our private branches on Stash, but in the interest of making it easier to switch between branches, we can put this into the "saas" branch for now.