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]
Disable locale checks to avoid warnings in Active Model tests [ci skip]

Missed AMo when adding to the other components in
ae196e85ee7169700afac2eecdc276bc06b10b8d.
Merge pull request #13463 from josevalim/jv-env

Do not store production information in .yml files
AC::Parameters#permit! permits hashes in array values
Unused classes in AV tests
Fix build failures related to the new ENV options in yml
Only build a ConnectionSpecification if required
Deprecate use of string in establish_connection as connection lookup
[ci skip] Update getting started Ruby version.
Merge pull request #13467 from JuanitoFatas/getting-started

[ci skip] Update getting started Ruby version.
Raise NoDatabaseError when db does not exist

Building on the work of #13427 this PR adds a helpful error message to the adapters: mysql, mysql2, and sqlite3
Ensure secret_key_base is set for all environments
Better missing `secret_key_base` error message

Previously the error was:

```
RuntimeError: You must set secret_key_base in your app's config
```

Will now be:


```
RuntimeError: Missing `secret_key_base` for 'production' environment, set this value in `config/secrets.yml`
```
Fix railties tests in master

Tests are failing due to missing env var on master https://travis-ci.org/rails/rails/jobs/15930622#L641

This adds an environment variable `ENV['RAILS_SECRET_KEY_BASE']` so these tests will pass.
Merge pull request #13469 from schneems/schneems/no-database-errors-all

Raise NoDatabaseError when db does not exist
Merge pull request #13471 from schneems/schneems/better-secrets-error-message

Better missing `secret_key_base` error message