Update INSTALL-ubuntu.md

This uses the new environment variables; by duplicating the sample config and suggesting similar changes. It's not exactly perfect as it still has the sample config problem, just to a much lesser extent. It may help to ship a "most commonly changed" configuration [db name + user/pass, hostname, smtp] that can be duplicated from and easily edited.

Follows up:
http://meta.discourse.org/t/environment-variables/11494/5
Update "check sample configuration"

... reflects having one discourse.conf.
Merge pull request #1754 from trident523/patch-1

Update INSTALL-ubuntu.md