Respect the max shared mem lists for postgresql.  Issue #597

This is based on this[1] patch to the open source server.  I looked at [2] for guidance, though it doesn't say much about what to do on the upper end.  It probably makes sense to have have some sort of hard ceiling, but I'm not sure where that should be.  I totally made up the 15% head room based on what % the old patch had.


[1] https://github.com/opscode/omnibus-chef/commit/ebd19785a1d65f10245aa96714b8d3dfc57de41f
[2] https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
Why default and not node? I believe it's possible for a user to have set that value in their chef-server.rb and we'll likely want to base our value off of that, not off the default set and node should do this.