Do not store production information in .yml files

Instead, read information from environment variables.
Guarantee the connection resolver handles string values

This commit also cleans up the rake tasks that were checking
for DATABASE_URL in different places.

In fact, it would be nice to deprecate DATABASE_URL usage in the long
term, considering the direction we are moving of allowing those in .yml
files.
Add examples and namespace ENV options with "RAILS_"
Use the new Resolver API in dbconsole
Update CHANGELOG [ci skip]
Merge pull request #13463 from josevalim/jv-env

Do not store production information in .yml files