Use Rails::Paths::Path#existent in database_configuration

Database configuration was trying to load the first path that
config.paths['config/database'] was returning even if the path didn't
exist in the filesystem.

Because Rails::Paths::Path has the possibility to return more than 1
path (as an array), database_configuration should filter down the paths
to the existing one and then load the first one.

This would make it possible to move the database.yml file and add the
new path to paths['config/database'] and still load the configurations.
Merge pull request #15015 from pothibo/database-configuration

Use Rails::Paths::Path#existent in database_configuration